tgsi_exec: Mark the store file default case as unreachable.

It isn't reached, and shouldn't be, and it's not like returning NULL
(instead of the &null case for TGSI_FILE_NULL) will do anything good.

Shaves another tiny bit of dead code off of release builds.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10605>

src/gallium/auxiliary/tgsi/tgsi_exec.c

@@ -1809,8 +1809,7 @@ store_dest_dstret(struct tgsi_exec_machine *mach,
       break;
 
    default:
-      assert( 0 );
-      return NULL;
+      unreachable("Bad destination file");
    }
 
    return dst;