pcap_mm: Fix compiler warnings

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

pcap_mm.c: In function ‘pcap_mm_read’:
pcap_mm.c:72:73: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
pcap_mm.c:79:72: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
pcap_io: Fix compiler warning

Fix the following compiler warning that occurs when building with "-W
-Wall -Wextra":

pcap_io.h: In function ‘pcap_prepare_header’:
pcap_io.h:628:255: warning: signed and unsigned type in conditional expression [-Wsign-compare]

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