undef $0 shouldn't warn about $0

RT #123910

    $ perl -we'undef $0'
    Use of uninitialized value $0 in undef operator at -e line 1.

Generally, undef should ignore its arg when determining which var was
undef: only magic will trigger an undef warning.

sv.c

@@ -15823,6 +15823,11 @@ S_find_uninit_var(pTHX_ const OP *const obase, const SV *const uninit_sv,
 
     switch (obase->op_type) {
 
+    case OP_UNDEF:
+        /* undef should care if its args are undef - any warnings
+         * will be from tied/magic vars */
+        break;
+
     case OP_RV2AV:
     case OP_RV2HV:
     case OP_PADAV:
@@ -16369,7 +16374,6 @@ S_find_uninit_var(pTHX_ const OP *const obase, const SV *const uninit_sv,
     case OP_ALARM:
     case OP_SEMGET:
     case OP_GETLOGIN:
-    case OP_UNDEF:
     case OP_SUBSTR:
     case OP_AEACH:
     case OP_EACH:

t/lib/warnings/9uninit

@@ -2186,3 +2186,11 @@ use warnings 'uninitialized';
 my $x = "" . open my $fh, "<", "no / such / file";
 EXPECT
 Use of uninitialized value in concatenation (.) or string at - line 3.
+########
+# RT #123910
+# undef's arg being undef doesn't trigger warnings - any warning will be
+# from tied/magic vars
+use warnings 'uninitialized';
+undef $0;
+EXPECT
+Use of uninitialized value in undef operator at - line 5.