mesa: do not pass NULL pointer to function not expecting NULLs

First argument for qsort() is declared to be never NULL, so ensure NULL
is never passed.

This has been detected by Undefined Behaviour Sanitizer (UBSan).

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29772>

src/mesa/program/program.c

@@ -414,6 +414,9 @@ _mesa_add_separate_state_parameters(struct gl_program *prog,
 {
    unsigned num_state_params = state_params->NumParameters;
 
+   if (num_state_params == 0)
+      return;
+
    /* All state parameters should be vec4s. */
    for (unsigned i = 0; i < num_state_params; i++) {
       assert(state_params->Parameters[i].Type == PROGRAM_STATE_VAR);