util/format: add missing null check in util_format_is_srgb()

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/11137
Fixes: ff6cf60cb8 ("gallium/util: add util_format_is_srgb() helper")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29160>

src/util/format/u_format.h

@@ -554,6 +554,12 @@ static inline bool
 util_format_is_srgb(enum pipe_format format)
 {
    const struct util_format_description *desc = util_format_description(format);
+
+   assert(desc);
+   if (!desc) {
+      return false;
+   }
+
    return desc->colorspace == UTIL_FORMAT_COLORSPACE_SRGB;
 }