dissector: ip_auth_hdr: Fix compiler warning

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

ip_authentication_hdr.c: In function ‘auth_hdr’:
proto_ip_authentication_hdr.c:57:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Also remove an unnecessary check for header length smaller than 0, since
it always results from an unsigned type and is now a size_t anyway.

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