Correct stupidities in 4c3ac4b

Allocating an extra SV for rare edge cases...er...only needs to be
done in those rare edge cases.

Uninitialized warnings are only supposed to be enabled when they are.

pp_hot.c

@@ -501,10 +501,13 @@ PP(pp_add)
     svl = TOPm1s;
 
     useleft = USE_LEFT(svl);
-    if(useleft && svr == svl) {
+    if(useleft && svr == svl && SvGMAGICAL(svl)) {
 	/* Print the uninitialized warning now, so it includes the vari-
 	   able name. */
-	if (!SvOK(svl)) report_uninit(svl), useleft = 0;
+	if (!SvOK(svl)) {
+	    if (ckWARN(WARN_UNINITIALIZED)) report_uninit(svl);
+	    useleft = 0;
+	}
 	/* Non-magical sv_mortalcopy */
 	svl = sv_newmortal();
 	sv_setsv_flags(svl, svr, 0);

t/lib/warnings/9uninit

@@ -694,6 +694,8 @@ sub FETCH { undef }
 
 tie my $m1, "";
 my $v = $m1 + $m1;
+no warnings;
+$v = $m1 + $m1;
 EXPECT
 Use of uninitialized value $m1 in addition (+) at - line 6.
 Use of uninitialized value $m1 in addition (+) at - line 6.