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).

Bug: http://b/32322088
Test: browser still works
Change-Id: Ied6662be567fb1bddc7ceb138cae1da77fb57976