gallium: do something sensible on the error path to try to avoid crashing in release builds

src/gallium/auxiliary/util/u_gen_mipmap.c

@@ -505,6 +505,9 @@ format_to_type_comps(enum pipe_format pformat,
       return;
    default:
       assert(0);
+      *datatype = UBYTE;
+      *comps = 0;
+      break;
    }
 }