regcomp.c: Guard against corrupting inversion list SV

I don't know of any cases where this happens, but in working on the next
commit I triggered a problem with shrinking an inversion list so much
that the required 0 UV at the beginning was freed.

embed.fnc

@@ -1529,7 +1529,7 @@ EiMRn	|bool	|invlist_is_iterating|NN SV* const invlist
 EiMRn	|IV*	|get_invlist_previous_index_addr|NN SV* invlist
 EiMn	|void	|invlist_set_previous_index|NN SV* const invlist|const IV index
 EiMRn	|IV	|invlist_previous_index|NN SV* const invlist
-EiMn	|void	|invlist_trim	|NN SV* const invlist
+EiMn	|void	|invlist_trim	|NN SV* invlist
 #endif
 EiMR	|SV*	|invlist_clone	|NN SV* const invlist
 EiMRn	|STRLEN*|get_invlist_iter_addr	|NN SV* invlist

proto.h

@@ -3667,7 +3667,7 @@ PERL_STATIC_INLINE IV	S_invlist_previous_index(SV* const invlist)
 PERL_STATIC_INLINE void	S_invlist_set_previous_index(SV* const invlist, const IV index);
 #define PERL_ARGS_ASSERT_INVLIST_SET_PREVIOUS_INDEX	\
 	assert(invlist)
-PERL_STATIC_INLINE void	S_invlist_trim(SV* const invlist);
+PERL_STATIC_INLINE void	S_invlist_trim(SV* invlist);
 #define PERL_ARGS_ASSERT_INVLIST_TRIM	\
 	assert(invlist)
 #  endif

regcomp.c

@@ -8382,15 +8382,18 @@ S_invlist_set_previous_index(SV* const invlist, const IV index)
 }
 
 PERL_STATIC_INLINE void
-S_invlist_trim(SV* const invlist)
+S_invlist_trim(SV* invlist)
 {
     PERL_ARGS_ASSERT_INVLIST_TRIM;
 
     assert(SvTYPE(invlist) == SVt_INVLIST);
 
     /* Change the length of the inversion list to how many entries it currently
-     * has */
-    SvPV_shrink_to_cur((SV *) invlist);
+     * has.  But don't shorten it so that it would free up the required
+     * initial 0 UV (and a trailing NUL byte) */
+    if (SvCUR(invlist) > TO_INTERNAL_SIZE(1) + 1) {
+        SvPV_shrink_to_cur(invlist);
+    }
 }
 
 #endif /* ifndef PERL_IN_XSUB_RE */