translate_generic: return NULL instead of assert(0) if format not supported

This gives the caller a chance to recover (or crash anyway otherwise).

src/gallium/auxiliary/translate/translate_generic.c

@@ -201,12 +201,6 @@ emit_B8G8R8A8_UNORM( const float *attrib, void *ptr)
    out[3] = TO_8_UNORM(attrib[3]);
 }
 
-static void 
-emit_NULL( const float *attrib, void *ptr )
-{
-   /* do nothing is the only sensible option */
-}
-
 static emit_func get_emit_func( enum pipe_format format )
 {
    switch (format) {
@@ -343,8 +337,7 @@ static emit_func get_emit_func( enum pipe_format format )
       return &emit_A8R8G8B8_UNORM;
 
    default:
-      assert(0); 
-      return &emit_NULL;
+      return NULL;
    }
 }
 
@@ -539,8 +532,12 @@ struct translate *translate_generic_create( const struct translate_key *key )
       tg->attrib[i].instance_divisor = key->element[i].instance_divisor;
 
       tg->attrib[i].emit = get_emit_func(key->element[i].output_format);
+      if(!tg->attrib[i].emit)
+      {
+         FREE(tg);
+         return NULL;
+      }
       tg->attrib[i].output_offset = key->element[i].output_offset;
-
    }
 
    tg->nr_attrib = key->nr_elements;