Fix incorrect error checking on unique_fd

The expression "!fd" calls the implicit conversion to int, but comparing
the raw fd against 0 won't work, since open() and other POSIX calls
returning file descriptors use -1 to signal an error.

Test: m verity_verifier
Change-Id: Ib117de62ff46c8d3389db54875bfa269fd539b51
1 file changed