release-1b4ea61: Fix sigismember usage

Summary:
`sigismember` returns -1 on error, so we should explicitly compare the
return value to 1 instead of assuming any non-zero value means success.

Reviewed By: gkmhub

Differential Revision: D15762039

fbshipit-source-id: 9e01a97358e204e4ddfb42712d67f67b82b4eacb