pcap_rw: Fix compiler warnings

Fix the following compiler warnings that occur when building with "-W
-Wall -Wextra":

pcap_rw.c: In function ‘pcap_rw_write’:
pcap_rw.c:30:6: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
pcap_rw.c: In function ‘pcap_rw_read’:
pcap_rw.c:50:6: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>