bugfix #2931 crash if $HOME not set

unchecked pointer returned from BLI_gethome().

source/blender/python/api2_2x/EXPP_interface.c

@@ -69,8 +69,15 @@ char *bpy_gethome(int append_scriptsdir)
 	else if (homedir[0] != '\0')
 		return homedir;
 
+	/* BLI_gethome() can return NULL if env vars are not set */
 	s = BLI_gethome();
 
+	if( !s )  /* bail if no $HOME */
+	{
+		printf("$HOME is NOT set\n");
+		return NULL;
+	}
+
 	if( strstr( s, ".blender" ) )
 		PyOS_snprintf( homedir, FILE_MAXDIR, s );
 	else