malloc_size() to get the true allocated space, as PERL_STRLEN_ROUNDUP()
might actually bump the request across an allocation size boundary.
p4raw-id: //depot/perl@33632
s = SvPVX_mutable(sv);
if (newlen > SvLEN(sv)) { /* need more room? */
-#ifndef MYMALLOC
+#ifndef Perl_safesysmalloc_size
newlen = PERL_STRLEN_ROUNDUP(newlen);
#endif
if (SvLEN(sv) && s) {