panfrost: fix crash when disassembling shader for trace

During disassembly triggered by PAN_MESA_DEBUG=trace,
the upper bits of the blend shader address are set from the passed
in frag_shader. However, this is 0 for some blend shaders. In this case,
skip the blend shader disassembly.

This fixes a failing assert at line 86 of panfrost/lib/genxml/decode.h.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30603>

src/panfrost/lib/genxml/decode.c

@@ -213,6 +213,12 @@ GENX(pandecode_blend)(struct pandecode_context *ctx, void *descs, int rt_no,
 #if PAN_ARCH >= 6
    if (b.internal.mode != MALI_BLEND_MODE_SHADER)
       return 0;
+   /* If we don't have a frag shader, we can't extract the LSB of the blend
+    * shader so return NULL in that case. It doesn't matter, because the
+    * blend shader won't be executed anyway, so disassembling is not
+    * super useful. */
+   if (!frag_shader)
+      return 0;
 
    return (frag_shader & 0xFFFFFFFF00000000ULL) | b.internal.shader.pc;
 #else