fix: use gmtime_r in get_timestamp for thread safety (#655)

gmtime(3) returns a pointer to a static internal buffer shared across
all threads. In a multi-threaded decoder, concurrent calls to
get_timestamp() could race on that buffer, corrupting decoded timestamps.

gmtime_r(3) takes a caller-supplied struct tm buffer, eliminating the
race. It is already used consistently elsewhere in the codebase
(log.c, waypoint.c, encode_aprs.c, beacon.c) and the portability shim
that makes it available on all platforms is in direwolf.h.

Add a null-return guard as the FIXME comment requested.

Fixes the FIXME at decode_aprs.c line 4041.

Author: chrisdebian

src/decode_aprs.c

@@ -4034,13 +4034,12 @@ time_t get_timestamp (decode_aprs_t *A, char *p)
 	}
 
 	struct tm *ptm;
-
+	struct tm tm_buf;
 	time_t ts;
 
 	ts = time(NULL);
-	// FIXME: use gmtime_r instead.
-	// Besides not being thread safe, gmtime could possibly return null.
-	ptm = gmtime(&ts);
+	ptm = gmtime_r(&ts, &tm_buf);
+	if (ptm == NULL) return ((time_t)0);
 
 	pdhm = (void *)p;
 	phms = (void *)p;