Check for localtime() failing.

It "shouldn't happen", but at least this squelches a Coverity complaint,
CID 1398224.

Change-Id: I9555f71a50574e9386a3c96d52143d838f7f121f
Reviewed-on: https://code.wireshark.org/review/21160
Reviewed-by: Guy Harris <guy@alum.mit.edu>

ui/text_import.c

@@ -921,6 +921,7 @@ text_import(text_import_info_t *info)
 {
     yyscan_t scanner;
     int ret;
+    struct tm *now_tm;
 
     packet_buf = (guint8 *)g_malloc(sizeof(HDR_ETHERNET) + sizeof(HDR_IP) +
                                     sizeof(HDR_SCTP) + sizeof(HDR_DATA_CHUNK) +
@@ -938,8 +939,17 @@ text_import(text_import_info_t *info)
     packet_start = 0;
     packet_preamble_len = 0;
     ts_sec = time(0);            /* initialize to current time */
-    /* We trust the OS not to provide a time before the Epoch. */
-    timecode_default = *localtime(&ts_sec);
+    now_tm = localtime(&ts_sec);
+    if (now_tm == NULL) {
+        /*
+         * This shouldn't happen - on UN*X, this should Just Work, and
+         * on Windows, it won't work if ts_sec is before the Epoch,
+         * but it's long after 1970, so....
+         */
+        fprintf(stderr, "localtime(right now) failed\n");
+        exit(-1);
+    }
+    timecode_default = *now_tm;
     timecode_default.tm_isdst = -1;     /* Unknown for now, depends on time given to the strptime() function */
     ts_usec = 0;