gl-renderer: Fix possible memory leak when no dmabuf modifers are supported

Some drivers support EGL_EXT_image_dma_buf_import_modifiers for format
enumeration, but don't have any modifiers. In this case, on platforms where
malloc(0) returns non-NULL, we would leak that allocation to the caller.

Handle this by noticing when the number of supported modifiers is 0 and
returning early.

libweston/renderer-gl/gl-renderer.c

@@ -2424,7 +2424,8 @@ gl_renderer_query_dmabuf_modifiers(struct weston_compositor *wc, int format,
 
 	if (!gr->has_dmabuf_import_modifiers ||
 		!gr->query_dmabuf_modifiers(gr->egl_display, format, 0, NULL,
-					    NULL, &num)) {
+					    NULL, &num) ||
+		num == 0) {
 		*num_modifiers = 0;
 		return;
 	}