Fix unsafe assumption that struct timeval.tv_sec is a "long".

It typically is a "long", but it seems possible that on some platforms
it wouldn't be.  In any case, this silences a compiler warning on
OpenBSD (cf buildfarm member curculio).

While at it, use snprintf not sprintf.  This format string couldn't
possibly overrun the supplied buffer, but that doesn't seem like
a good reason not to use the safer style.

Oversight in commit f828654e.  Back-patch to 9.6 where that came in.

src/backend/utils/error/elog.c

@@ -2484,7 +2484,8 @@ log_line_prefix(StringInfo buf, ErrorData *edata)
 						saved_timeval_set = true;
 					}
 
-					sprintf(strfbuf, "%ld.%03d", saved_timeval.tv_sec,
+					snprintf(strfbuf, sizeof(strfbuf), "%ld.%03d",
+							 (long) saved_timeval.tv_sec,
 							 (int) (saved_timeval.tv_usec / 1000));
 
 					if (padding != 0)