Prevent @argv and <> from conflicting.

Issue Perl#19340: This prevents an obscure segfault.

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

doio.c

@@ -1393,8 +1393,14 @@ Perl_nextargv(pTHX_ GV *gv, bool nomagicopen)
         return NULL;
     while (av_count(GvAV(gv)) > 0) {
         STRLEN oldlen;
+
+        /* Leave this SV unfreed. Most of the time it will leak,
+         * but as the number of args is likely small that shouldn't
+         * matter much. More importantly, this avoids a double-free
+         * in the event that the SV ends up assigned to $_.
+         */
         SV *const sv = av_shift(GvAV(gv));
-        SAVEFREESV(sv);
+
         SvTAINTED_off(GvSVn(gv)); /* previous tainting irrelevant */
         sv_setsv(GvSVn(gv),sv);
         SvSETMAGIC(GvSV(gv));

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;