Fix two undefined behaviors for the pcap_loop() call

Fix two undefined behaviors for the pcap_loop() call

Limit the --skip argument to INT_MAX.
Limit the sum of -c and --skip arguments to INT_MAX.

Fix the regression in 3eab64d3: The '--skip 0' option is allowed to get
the first packet in some loop e.g. in a shell script.

The errors were:

tcpdump.c:2696:8: runtime error: implicit conversion from type 'int' of
  value -1 (32-bit, signed) to type 'u_int' (aka 'unsigned int')
  changed the value to 4294967295 (32-bit, unsigned)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior tcpdump.c:2696:8

tcpdump.c:2696:8: runtime error: implicit conversion from type 'u_int'
  (aka 'unsigned int') of value 4294967295 (32-bit, unsigned) to type
  'int' changed the value to -1 (32-bit, signed)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior tcpdump.c:2696:8