avoid kdump crash on localtime() failure

[Jonathan Gray]

So now time is printed by default afl has found that time_t values
such as -9223372035438150153 will cause localtime() to fail and
return NULL.  strftime() can't deal with this and will at some point
dereference tm without checking if it is NULL causing a crash.

Index: ktrstruct.c
===================================================================
RCS file: /cvs/src/usr.bin/kdump/ktrstruct.c,v
retrieving revision 1.8
diff -u -p -r1.8 ktrstruct.c
--- ktrstruct.c 15 Dec 2014 01:48:54 -0000      1.8
+++ ktrstruct.c 15 Dec 2014 13:56:03 -0000
@@ -146,8 +146,10 @@ print_time(time_t t, int relative, int h
 
        if (!relative) {
                tm = localtime(&t);
-               (void)strftime(timestr, sizeof(timestr), TIME_FORMAT, tm);
-               printf("<\"%s\">", timestr);
+               if (tm != NULL) {
+                       (void)strftime(timestr, sizeof(timestr), TIME_FORMAT, 
tm);
+                       printf("<\"%s\">", timestr);
+               }
        }
 }