Fix code mistake picked up by coverity.

Both sides of the conditional && were evaluting the same thing: "fdflags &
QIO_FDFLAG_WRITEABLE.". The comment suggests that it should be checking for
readability and writeability.

See CID 1212287
Merge branch 'master' of https://github.com/chapel-lang/chapel into coverity-1212287
Merge pull request #943 from thomasvandoren/coverity-1212287

Fix code mistake picked up by coverity (1212287).

Both sides of the conditional && were evaluting the same thing: "fdflags &
QIO_FDFLAG_WRITEABLE.". The comment suggests that it should be checking for
readability and writeability.