wcrtomb returns a size_t, so it's silly to immediately assign it into
an int variable. Apparently running gcc with LTO enabled points this
out as an error.
This was benign as far as I can see: the obvious risk of integer
overflow could only happen if the OS wanted to convert a single wide
character into more than 2^31 bytes, and the test of the return value
against (size_t)-1 for an error check seems to work anyway in
practice, although I suspect that's only because of implementation-
defined behaviour in gcc at the point where the size_t is narrowed to
a signed int.
(cherry picked from commit 99f5fa34ab918894939c8aca00e44730223c0e54)
d527b1a886