pan/midgard: Allow NULL argument in mir_has_arg

It's sometimes convenient to call this with no instruction specified. By
definition, a missing instruction cannot reference any argument, so
let's check for NULL and shortciruit to false.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>

src/panfrost/midgard/compiler.h

@@ -566,6 +566,9 @@ v_mov(unsigned src, midgard_vector_alu_src mod, unsigned dest)
 static inline bool
 mir_has_arg(midgard_instruction *ins, unsigned arg)
 {
+        if (!ins)
+                return false;
+
         for (unsigned i = 0; i < ARRAY_SIZE(ins->src); ++i) {
                 if (ins->src[i] == arg)
                         return true;