Crash when save executed without arg

[twetzel59]

If the save command is missing an argument, NimEdit crashes with an IOError. It tries to open the directory.

nimedit.nim(1203)        nimedit
nimedit.nim(1156)        mainProc
nimedit.nim(1013)        processEvents
nimedit.nim(782)         runAction
prompt.nim(273)          runCmd
buffer.nim(506)          saveAs
buffer.nim(495)          save
buffer.nim(472)          saveAsTemp
system.nim(2839)         sysFatal
Error: unhandled exception: cannot open: /home/twetzel59/Projects/nimedit/ [IOError]