The function fwscanf() return value is wrong when encountering an early
matching failure.

According to the Open Group documentation for fwscanf:
"Upon successful completion, these functions shall return the number of
successfully matched and assigned input items; this number can be zero in
the event of an early matching failure."

Without this change, fwscanf would return EOF in the case of an early
matching failure, instead of the proper return value of 0.

This change aligns fwscanf(3) with the implementation in fscanf(3).

PR: 202240
Submitted by: rajendra.sy@gmail.com
Reviewed by: jhb, cem
Approved by: sjg (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D13288