hv_undef_flags: eliminate spurious -Warray-bounds warning

Eliminates:

In file included from perl.h:6205,
                 from hv.c:35:
hv.c: In function ‘Perl_hv_undef_flags’:
hv.h:460:26: warning: array subscript [0, 9223372036854775807] is outside array bounds of ‘char[0]’ [-Warray-bounds]
  460 | #define HEK_FLAGS(hek)  (*((unsigned char *)(HEK_KEY(hek))+HEK_LEN(hek)+1))
      |                         ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
embed.h:292:78: note: in definition of macro ‘hv_common’
  292 | mmon(a,b,c,d,e,f,g,h)             Perl_hv_common(aTHX_ a,b,c,d,e,f,g,h)
      |                                                                ^

hv.h:474:34: note: in expansion of macro ‘HEK_FLAGS’
  474 | #define HEK_UTF8(hek)           (HEK_FLAGS(hek) & HVhek_UTF8)
      |                                  ^~~~~~~~~
hv.h:579:55: note: in expansion of macro ‘HEK_UTF8’
  579 |     hv_common((hv), NULL, HEK_KEY(hek), HEK_LEN(hek), HEK_UTF8(hek), \
      |                                                       ^~~~~~~~
hv.c:2268:19: note: in expansion of macro ‘hv_deletehek’
 2268 |             (void)hv_deletehek(PL_stashcache, HvNAME_HEK(hv), G_DISCARD);
      |                   ^~~~~~~~~~~~

This does appear to be spurious, the gcc documentation claims not
to check array bounds on trailing 1 element arrays for backward
compatibility.

This warning would be reasonable if the hek itself was allocated on
the stack, part of an array of HEKs, or embedded in another struct,
but HvNAME_HEK() returns a pointer to a HEK, for which gcc has no
knowledge of the source.

This warning only appears in debugging builds, but the only
difference from non-debugging builds is the HEKfARG() within the
DEBUG_o(), which is a simple cast to (void *), which I don't see
having an effect on whether the compiler considers HEK itself as
part of a structure, or of unknown origin.

So I expect it's just a gcc bug, and there are know issues with this
warning.

hv.c

@@ -2263,9 +2263,10 @@ Perl_hv_undef_flags(pTHX_ HV *hv, U32 flags)
      * in sv_clear(), and changes here should be done there too */
     if (PL_phase != PERL_PHASE_DESTRUCT && HvHasNAME(hv)) {
         if (PL_stashcache) {
+            HEK *hek = HvNAME_HEK(hv);
             DEBUG_o(Perl_deb(aTHX_ "hv_undef_flags clearing PL_stashcache for '%"
-                             HEKf "'\n", HEKfARG(HvNAME_HEK(hv))));
-            (void)hv_deletehek(PL_stashcache, HvNAME_HEK(hv), G_DISCARD);
+                             HEKf "'\n", HEKfARG(hek)));
+            (void)hv_deletehek(PL_stashcache, hek, G_DISCARD);
         }
         hv_name_set(hv, NULL, 0, 0);
     }