make MULTICALL safe across cxstack reallocs

[perl #115602]
MUTLICALL sets a local var, cx, to point to the current context stack
frame. When a function is called, the context stack might be realloc()ed,
in which case cx would point to freed memory.

cop.h

@@ -1217,7 +1217,8 @@ See L<perlcall/LIGHTWEIGHT CALLBACKS>.
 
 #define POP_MULTICALL \
     STMT_START {							\
-	if (! ((CvDEPTH(multicall_cv) = cx->blk_sub.olddepth)) ) {	\
+	cx = &cxstack[cxstack_ix];					\
+        if (! ((CvDEPTH(multicall_cv) = cx->blk_sub.olddepth)) ) {	\
 		LEAVESUB(multicall_cv);					\
 	}								\
 	POPBLOCK(cx,PL_curpm);						\

ext/XS-APItest/t/multicall.t

@@ -7,7 +7,7 @@
 use warnings;
 use strict;
 
-use Test::More tests => 6;
+use Test::More tests => 7;
 use XS::APItest;
 
 
@@ -48,3 +48,16 @@ use XS::APItest;
     is($destroyed, 1, "f now destroyed");
 
 }
+
+# [perl #115602]
+# deep recursion realloced the CX stack, but the dMULTICALL local var
+# 'cx' still pointed to the old one.
+# Thius doesn;t actually test the failure (I couldn't think of a way to
+# get the failure to show at the perl level) but it allows valgribnd or
+# similar to spot any errors.
+
+{
+    sub rec { my $c = shift; rec($c-1) if $c > 0 };
+    my @r = XS::APItest::multicall_each { rec(90) } 1,2,3;
+    pass("recursion");
+}