Panic when BuildsFrom does not find an action with the Payload correctly set

[fdonzello]

Hi,

as in the issue title..

Everything works fine when adding the Payload to the action design.

Maybe I can send a PR, or someone else who is very confident with the project can.. I'm pretty new to goa but I think that we can show a friendly error message here.

The error:

goagen bootstrap -d blabla/design
exit status 2
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x13a3942]

goroutine 1 [running]:
myproject.com/vendor/github.com/goadesign/gorma/dsl.Payload(0x13e6cc0, 0xc42025f9a0, 0x1478069, 0x4)
	/Users/myuser/Work/web/go/src/myproject.com/vendor/github.com/goadesign/gorma/dsl/relationalmodel.go:152 +0xf2
myproject.com/design.glob..func5.1.1.1()
	/Users/myuser/Work/web/go/src/myproject.com/design/models.go:14 +0x7d
myproject.com/vendor/github.com/goadesign/goa/dslengine.Execute(0x14aa338, 0x16a00a0, 0xc4202971a0, 0x16a00a0)
	/Users/myuser/Work/web/go/src/myproject.com/vendor/github.com/goadesign/goa/dslengine/runner.go:132 +0x93
myproject.com/vendor/github.com/goadesign/goa/dslengine.runSet(0xc42025f990, 0x1, 0x1, 0x0, 0x0)
	/Users/myuser/Work/web/go/src/myproject.com/vendor/github.com/goadesign/goa/dslengine/runner.go:298 +0x17a
myproject.com/vendor/github.com/goadesign/gorma.(*StorageGroupDefinition).IterateSets.func1.1.2(0xc4202971a0, 0x0, 0x0)
	/Users/myuser/Work/web/go/src/myproject.com/vendor/github.com/goadesign/gorma/storagegroup.go:90 +0x87
myproject.com/vendor/github.com/goadesign/gorma.(*RelationalModelDefinition).IterateBuildSources(0xc420212ea0, 0xc4201fbc10, 0x0, 0x0)
	/Users/myuser/Work/web/go/src/myproject.com/vendor/github.com/goadesign/gorma/relationalmodel.go:174 +0x58
myproject.com/vendor/github.com/goadesign/gorma.(*StorageGroupDefinition).IterateSets.func1.1(0xc420212ea0, 0xc420296f00, 0x1477be1)
	/Users/myuser/Work/web/go/src/myproject.com/vendor/github.com/goadesign/gorma/storagegroup.go:92 +0xf9
myproject.com/vendor/github.com/goadesign/gorma.(*RelationalStoreDefinition).IterateModels(0xc4202886c0, 0xc4201fbd48, 0x1, 0x0)
	/Users/myuser/Work/web/go/src/myproject.com/vendor/github.com/goadesign/gorma/relationalstore.go:51 +0x1c9
myproject.com/vendor/github.com/goadesign/gorma.(*StorageGroupDefinition).IterateSets.func1(0xc4202886c0, 0xc420296b70, 0x147a2ec)
	/Users/myuser/Work/web/go/src/myproject.com/vendor/github.com/goadesign/gorma/storagegroup.go:95 +0xbe
myproject.com/vendor/github.com/goadesign/gorma.(*StorageGroupDefinition).IterateStores(0xc42024ca80, 0xc4201fbe68, 0x1, 0x0)
	/Users/myuser/Work/web/go/src/myproject.com/vendor/github.com/goadesign/gorma/storagegroup.go:35 +0x1d2
myproject.com/vendor/github.com/goadesign/gorma.(*StorageGroupDefinition).IterateSets(0xc42024ca80, 0x14aa3a0)
	/Users/myuser/Work/web/go/src/myproject.com/vendor/github.com/goadesign/gorma/storagegroup.go:97 +0xb5
myproject.com/vendor/github.com/goadesign/goa/dslengine.Run(0x16a16e0, 0xc4202652e0)
	/Users/myuser/Work/web/go/src/myproject.com/vendor/github.com/goadesign/goa/dslengine/runner.go:97 +0x103
main.main()
	/Users/myuser/Work/web/go/src/myproject.com/goagen859935429/main.go:25 +0x5a

go env:

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/myuser/Work/web/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/z4/pz3l5h_s7m775dq8y480th1r0000gn/T/go-build343316665=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

[bketelsen]

would love a PR, I'm traveling solid for the next month won't be able to look at this for quite a while!

[fdonzello]

I really hope to make it!