crtend: accurately check for the start of .ctors

For the hypothetic situation where crtbegin.o is not linked into the
binary, but crtend.o is, which results in the missing starting sentinel
in the ctors array, be careful to not iterate past the start of the
section.

Reviewed by: andrew, dim
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D48700