Work around perl bug in SvPVutf8().

Certain things like typeglobs or readonly things like $^V cause
perl's SvPVutf8() to die nastily and crash the backend. To avoid
that bug we make a copy of the object, which will subsequently be
garbage collected.

Back patched to 9.1 where we first started using SvPVutf8().

Per -hackers discussion. Original problem reported by David Wheeler.

src/pl/plperl/plperl_helpers.h

@@ -50,8 +50,14 @@ sv2cstr(SV *sv)
 
 	/*
 	 * get a utf8 encoded char * out of perl. *note* it may not be valid utf8!
+	 *
+	 * SvPVutf8() croaks nastily on certain things, like typeglobs and
+	 * readonly object such as $^V. That's a perl bug - it's not supposed to
+	 * happen. To avoid crashing the backend, we make a mortal copy of the
+	 * sv before passing it to SvPVutf8(). The copy will be garbage collected
+	 * very soon (see perldoc perlguts).
 	 */
-	val = SvPVutf8(sv, len);
+	val = SvPVutf8(sv_mortalcopy(sv), len);
 
 	/*
 	 * we use perls length in the event we had an embedded null byte to ensure