[libc] fix line buffered empty file writes

Previously, any line buffered write of size 0 would cause an error.
The variable used to track the index of the last newline started at
the size of the write - 1, which underflowed. Now it's handled properly,
and a test has been added to prevent regressions.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D127914