dump: check lseek() return in pcap record-counting loop

When --dump-file points to an existing pcap file, dump_init_file()
counts the existing records by reading each pcap record header and
seeking past the payload.  The lseek() return value was not checked:
if it fails (e.g. EOVERFLOW on a 32-bit build with a very large
incl_len, or any other seek error) the file position does not advance,
the next read_write() call reads the same header again, and the loop
spins forever — hanging daemon startup.

Fix: break out of the loop on lseek failure.  An undercount of
existing records is harmless; a hanging startup is not.

Signed-off-by: Dominik <dl6er@dl6er.de>

src/dump.c

@@ -85,7 +85,8 @@ void dump_init(void)
       /* count existing records */
       while (read_write(daemon->dumpfd, (void *)&pcap_header, sizeof(pcap_header), RW_READ))
 	{
-	  lseek(daemon->dumpfd, pcap_header.incl_len, SEEK_CUR);
+	  if (lseek(daemon->dumpfd, pcap_header.incl_len, SEEK_CUR) == (off_t)-1)
+	    break;
 	  packet_count++;
 	}
     }