fix magic and upgraded SV handling for setsockopt()'s OPTVAL

The code here checked SV flags before fetching magic, potentially
getting confused if magic fetched changed flags.

This also fixes handling for upgraded SVs, but I'm not sure that can
be tested sufficiently portably.

pp_sys.c

@@ -2707,13 +2707,14 @@ PP(pp_ssockopt)
     case OP_SSOCKOPT: {
             const char *buf;
             int aint;
+            SvGETMAGIC(sv);
             if (SvPOKp(sv)) {
                 STRLEN l;
-                buf = SvPV_const(sv, l);
+                buf = SvPVbyte_nomg(sv, l);
                 len = l;
             }
             else {
-                aint = (int)SvIV(sv);
+                aint = (int)SvIV_nomg(sv);
                 buf = (const char *) &aint;
                 len = sizeof(int);
             }

t/io/socket.t

@@ -281,6 +281,34 @@ SKIP: {
     ), "0\n", {}, "fresh socket not inherited across exec");
 }
 
+SKIP:
+{
+    my $val;
+    {
+        package SetsockoptMagic;
+        sub TIESCALAR { bless {}, shift }
+        sub FETCH { $val }
+    }
+    # setsockopt() magic
+    socket(my $sock, PF_INET, SOCK_STREAM, $tcp);
+    $val = 0;
+    # set a known value
+    ok(setsockopt($sock, SOL_SOCKET, SO_REUSEADDR, 1),
+       "set known SO_REUSEADDR");
+    isnt(getsockopt($sock, SOL_SOCKET, SO_REUSEADDR), pack("i", 0),
+       "check that worked");
+    tie my $m, "SetsockoptMagic";
+    # trigger the magic with the value 0
+    $val = pack("i", 0);
+    my $temp = $m;
+
+    $val = 1;
+    ok(setsockopt($sock, SOL_SOCKET, SO_REUSEADDR, $m),
+       "set SO_REUSEADDR from magic");
+    isnt(getsockopt($sock, SOL_SOCKET, SO_REUSEADDR), pack("i", 0),
+       "check SO_REUSEADDR set correctly");
+}
+
 done_testing();
 
 my @child_tests;