Merge musl wcsrtombs fix.

test/input.output/file.streams/fstreams/filebuf.virtuals/seekoff.pass.cpp
was failing due to wcsrtombs() not doing the right thing with partial
conversions on musl.  The musl folks fixed this within a few hours of
reporting the problem.  This merges the fix:
http://git.musl-libc.org/cgit/musl/commit/src/multibyte/wcsrtombs.c?id=8fba4458afb7304b32ca887e4a3990c6029131f9

Change-Id: If9e401deef767f7c5585ecfe904a9a83ab525e10