Crash on malformed debug draw commands

[slipher]

When handling debug draw commands, the renderer may access an invalid tess.verts pointer if the debug draw commands fail to issue a BEGIN command. E.g.

// from the sgame
DebugDrawQuake dd;
vec3_t p{};
dd.vertex(p, -1);
dd.vertex(p, -1);
dd.sendCommands();