Uninitialized tmbuf.

Fix for Coverity perl5 CID 29088: Uninitialized scalar variable (UNINIT)
uninit_use: Using uninitialized value tmbuf.tm_year.

There is a code path that can lead to accessing uninitialized tmbuf:
when the too-small or too-large time inputs to gmtime/localtime
happen.

- make it so that the tm_year is used only on successful code path:
  pp_sys.c
- add the gmtime failed / localtime failed errors to perldiag:
  pod/perldiag.pod
- test those errors: t/op/time.t

pod/perldiag.pod

@@ -2121,6 +2121,11 @@ a term, so it's looking for the corresponding right angle bracket, and
 not finding it.  Chances are you left some needed parentheses out
 earlier in the line, and you really meant a "less than".
 
+=item gmtime(%f) failed
+
+(W overflow) You called C<gmtime> with a number that it could not handle:
+too large, too small, or NaN.  The returned value is C<undef>.
+
 =item gmtime(%f) too large
 
 (W overflow) You called C<gmtime> with a number that was larger than
@@ -2780,6 +2785,11 @@ L<perlfunc/listen>.
 form of C<open> does not support pipes, such as C<open($pipe, '|-', @args)>.
 Use the two-argument C<open($pipe, '|prog arg1 arg2...')> form instead.
 
+=item localtime(%f) failed
+
+(W overflow) You called C<localtime> with a number that it could not handle:
+too large, too small, or NaN.  The returned value is C<undef>.
+
 =item localtime(%f) too large
 
 (W overflow) You called C<localtime> with a number that was larger

pp_sys.c

@@ -4485,30 +4485,29 @@ PP(pp_gmtime)
     }
 
     if (err == NULL) {
+	/* diag_listed_as: gmtime(%f) failed */
 	/* XXX %lld broken for quads */
 	Perl_ck_warner(aTHX_ packWARN(WARN_OVERFLOW),
 		       "%s(%.0" NVff ") failed", opname, when);
     }
 
     if (GIMME != G_ARRAY) {	/* scalar context */
-	SV *tsv;
-	/* XXX newSVpvf()'s %lld type is broken, so cheat with a double */
-	double year = (double)tmbuf.tm_year + 1900;
-
         EXTEND(SP, 1);
         EXTEND_MORTAL(1);
 	if (err == NULL)
 	    RETPUSHUNDEF;
-
-	tsv = Perl_newSVpvf(aTHX_ "%s %s %2d %02d:%02d:%02d %.0f",
-			    dayname[tmbuf.tm_wday],
-			    monname[tmbuf.tm_mon],
-			    tmbuf.tm_mday,
-			    tmbuf.tm_hour,
-			    tmbuf.tm_min,
-			    tmbuf.tm_sec,
-			    year);
-	mPUSHs(tsv);
+       else {
+           mPUSHs(Perl_newSVpvf(aTHX_ "%s %s %2d %02d:%02d:%02d %.0f",
+                                dayname[tmbuf.tm_wday],
+                                monname[tmbuf.tm_mon],
+                                tmbuf.tm_mday,
+                                tmbuf.tm_hour,
+                                tmbuf.tm_min,
+                                tmbuf.tm_sec,
+                                /* XXX newSVpvf()'s %lld type is broken,
+                                 * so cheat with a double */
+                                (double)tmbuf.tm_year + 1900));
+        }
     }
     else {			/* list context */
 	if ( err == NULL )

t/op/time.t

@@ -6,7 +6,7 @@ BEGIN {
     require './test.pl';
 }
 
-plan tests => 66;
+plan tests => 70;
 
 # These tests make sure, among other things, that we don't end up
 # burning tons of CPU for dates far in the future.
@@ -210,7 +210,7 @@ SKIP: { #rt #73040
 	|| $small_time == $smallest
         || $big_time - 200 != $biggest
 	|| $big_time == $biggest) {
-	skip "Can't represent test values", 4;
+	skip "Can't represent test values", 8;
     }
     my $small_time_f = sprintf("%.0f", $small_time);
     my $big_time_f = sprintf("%.0f", $big_time);
@@ -221,17 +221,20 @@ SKIP: { #rt #73040
     $warning = '';
     my $date = gmtime($big_time);
     like $warning, qr/^gmtime\($big_time_f\) too large/;
+    like $warning, qr/^gmtime\($big_time_f\) failed/m;
 
     $warning = '';
     $date = localtime($big_time);
     like $warning, qr/^localtime\($big_time_f\) too large/;
+    like $warning, qr/^localtime\($big_time_f\) failed/m;
 
     $warning = '';
     $date = gmtime($small_time);
     like $warning, qr/^gmtime\($small_time_f\) too small/;
+    like $warning, qr/^gmtime\($small_time_f\) failed/m;
 
     $warning = '';
     $date = localtime($small_time);
     like $warning, qr/^localtime\($small_time_f\) too small/;
-
+    like $warning, qr/^localtime\($small_time_f\) failed/m;
 }