Make map and grep preserve their args lists.

Issue Perl#19340: Previously if you mapped/grepped over an array and mutated
the array within the map/grep, a segfault would happen. This fixes that
by bumping the map/grep args' reference count at the start of the map/grep,
then enqueueing those args for a refcount decrement at the end.

MANIFEST

@@ -6166,6 +6166,7 @@ t/re/uniprops09.t		Test unicode \p{} regex constructs
 t/re/uniprops10.t		Test unicode \p{} regex constructs
 t/re/user_prop_race_thr.t	Test races in user-defined \p{} under threads
 t/README			Instructions for regression tests
+t/run/argv_free.t		Ensure no conflicts between @ARGV and <>.
 t/run/cloexec.t			Test close-on-exec.
 t/run/dtrace.pl			For dtrace.t
 t/run/dtrace.t			Test for DTrace probes

pp_ctl.c

@@ -972,17 +972,29 @@ PP(pp_grepstart)
     dSP;
     SV *src;
 
-    if (PL_stack_base + TOPMARK == SP) {
+    int count = SP - (PL_stack_base + TOPMARK);
+
+    if (count == 0) {
         (void)POPMARK;
         if (GIMME_V == G_SCALAR)
             XPUSHs(&PL_sv_zero);
         RETURNOP(PL_op->op_next->op_next);
     }
+
     PL_stack_sp = PL_stack_base + TOPMARK + 1;
     Perl_pp_pushmark(aTHX);				/* push dst */
     Perl_pp_pushmark(aTHX);				/* push src */
     ENTER_with_name("grep");					/* enter outer scope */
 
+    /* This prevents the map/grep arguments from disappearing midstream.
+       (e.g., map { @foo = () } @foo)
+     */
+    for (int i=0; i<count; i++) {
+        SV* sv = PL_stack_base[TOPMARK + i];
+        SvREFCNT_inc(sv);
+        SAVEFREESV(sv);
+    }
+
     SAVETMPS;
     SAVE_DEFSV;
     ENTER_with_name("grep_item");					/* enter inner scope */

t/op/grep.t

@@ -10,7 +10,7 @@ BEGIN {
     set_up_inc( qw(. ../lib) );
 }
 
-plan( tests => 67 );
+plan( tests => 68 );
 
 {
     my @lol = ([qw(a b c)], [], [qw(1 2 3)]);
@@ -238,3 +238,10 @@ pass 'no double frees with grep/map { undef *_ }';
     my @a = map { 1; "$_" } 1,2;
     is("@a", "1 2", "PADTMP");
 }
+
+{
+    # Ensure that the map args list doesn't disappear midstream:
+    my @foo = qw(1 2 3 4 5 6 7);
+    my @foo2 = map { @foo = (); $_ } @foo;
+    is("@foo2", "1 2 3 4 5 6 7", 'map preserves args list');
+}

t/run/argv_free.t

@@ -0,0 +1,13 @@
+#!./perl
+
+BEGIN {
+    chdir 't' if -d 't';
+    @INC = '../lib';
+    require './test.pl';
+    skip_all_without_config('d_fcntl');
+}
+
+system $^X, '-e', 'close STDIN; map <>, @ARGV', 1, 2;
+is($?, 0, '@ARGV does not conflict with <>');
+
+done_testing;