CVE-2017-13039/IKEv1: Do more bounds checking.

Have ikev1_attrmap_print() and ikev1_attr_print() do full bounds
checking, and return null on a bounds overflow.  Have their callers
check for a null return.

This fixes a buffer over-read discovered by Bhargava Shastry,
SecT/TU Berlin.

Add a test using the capture file supplied by the reporter(s), modified
so the capture file won't be rejected as an invalid capture.