main: silence missing return value warning in array_index_of_resource()

v2: return -1 instead of 0, per Emil Velikov.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/mesa/main/shader_query.cpp

@@ -537,6 +537,7 @@ array_index_of_resource(struct gl_program_resource *res,
       return get_matching_index(RESOURCE_VAR(res), name);
    default:
       assert(!"support for resource type not implemented");
+      return -1;
    }
 }