doio: fix shmio args assert

PERL_ARGS_ASSERT_DO_SHMIO is equivalent to `assert(mark); assert(sp)`.
There is no point in checking pointers that have already been
dereferenced by previous statements. An optimizing compiler will simply
remove those asserts as no-ops.

Move the asserts up to before any of the function arguments are used.

Author: mauke

doio.c

@@ -3356,16 +3356,16 @@ I32
 Perl_do_shmio(pTHX_ I32 optype, SV **mark, SV **sp)
 {
 #ifdef HAS_SHM
+    PERL_ARGS_ASSERT_DO_SHMIO;
+    PERL_UNUSED_ARG(sp);
+
     char *shm;
     struct shmid_ds shmds;
     const I32 id = SvIVx(*++mark);
     SV * const mstr = *++mark;
     const I32 mpos = SvIVx(*++mark);
     const I32 msize = SvIVx(*++mark);
 
-    PERL_ARGS_ASSERT_DO_SHMIO;
-    PERL_UNUSED_ARG(sp);
-
     SETERRNO(0,0);
     if (shmctl(id, IPC_STAT, &shmds) == -1)
         return -1;