v3d: avoid 0-size variable length array

Declaring a variable-length array (VLA) based on a variable that can be
0 is declared dangerous.

In this case, the variable can't take value 0, so adding an assertion
fixes the issue.

This was detected by static analyzer.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32819>

src/broadcom/compiler/vir_register_allocate.c

@@ -267,6 +267,8 @@ get_spill_type_for_temp(struct v3d_compile *c, int temp)
 static int
 v3d_choose_spill_node(struct v3d_compile *c)
 {
+        assert(c->num_temps > 1);
+
         const float tmu_scale = 10;
         float block_scale = 1.0;
         float spill_costs[c->num_temps];