Check for bad packets in getaddrinfo.c's getanswer.

The near duplicate in gethnamaddr.c was already doing so (this fix
is basically copy and pasted from there, but with both copies modified
to avoid skirting undefined behavior).

(cherrypick of 27a4459d from master.)

Bug: http://b/32322088
Change-Id: I9668b1515ad87665e47cae962812474bb480f205