androiddump: Fix tcpdump op prec bug with used_buffer_length calc

androiddump reads data from tcpdump in chunks that not necessarily
match packet sizes. Whenever a partly read packet is found in buffers
more data is read and the number of used bytes in buffer is updated.
The calculation of used_buffer_length is wrong though leading to most
packets are read and thrown away.

Problem fixed by correcting an operator precedence mistake where
used_buffer_length would only be increaded with 0 or 1 insted of
actual number of read bytes.

Change-Id: Ib5c4e1d96e7ba499f26796ffa5d7ce6a116b9a6e
Reviewed-on: https://code.wireshark.org/review/20666
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>

extcap/androiddump.c

@@ -2644,7 +2644,7 @@ static int capture_android_wifi_tcpdump(char *interface, char *fifo,
                 return EXIT_CODE_GENERIC;
             }
 
-            if (used_buffer_length += length > 1) {
+            if ((used_buffer_length += length) > 1) {
                 break;
             }
         }