Fix bug #1356: .obj loading bug in picolib

There's a bug in picolib when parsing the first group in an .obj file.

The library tries to set the surface name to a surface pointer
that is still NULL.

This gets silently ignored but the problem reappears later
in a warning message when the material or a face is parsed.

libs/picomodel/pm_obj.c

@@ -676,7 +676,7 @@ static picoModel_t *_obj_load( PM_PARAMS_LOAD ){
 #endif
 			}
 
-			if ( curFace == 0 ) {
+			if ( curFace == 0 && curSurface != NULL ) {
 				PicoSetSurfaceName( curSurface,groupName );
 			}
 			else