mesa: clear TransformFeedback.NumVarying on error

The memory is freed before trying to reallocate it.
If the alloc fails, NumVarying should be updated to 0.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37331>

src/mesa/main/transformfeedback.c

@@ -884,6 +884,7 @@ transform_feedback_varyings(struct gl_context *ctx,
 
    if (!shProg->TransformFeedback.VaryingNames) {
       _mesa_error(ctx, GL_OUT_OF_MEMORY, "glTransformFeedbackVaryings()");
+      shProg->TransformFeedback.NumVarying = 0;
       return;
    }