Prevent recursive MSan interceptors in fgets(3)

Summary:
This corrects intercepting this call on NetBSD without false positives.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, eugenis, vitalybuka

Reviewed By: vitalybuka

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D42884

llvm-svn: 325207