binutils: Fix bugs found by -Wpointer-compare

The MIPS bug was introduced by upstream commit 7403cb630, which failed
to account for the additional indirection introduced and also dropped
one of the checks; change it to the standard "NULL-or-empty" check as
used elsewhere in BFD, which is also what upstream now has.

Submitted by: James Clarke <jrtc27@jrtc27.com>
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D21911