unix: don't convert stat buffer when syscall fails
unix: don't convert stat buffer when syscall fails

Don't call uv__to_stat() when the stat/fstat/lstat system call fails;
the stack-allocated buffer contains only garbage in that case.

Not a very serious bug it's technically undefined behavior and it made
valgrind squawk.

Introduced in commit 499c7976 ("unix, windows: nanosecond resolution
for uv_fs_[fl]stat").

PR-URL: https://github.com/libuv/libuv/pull/921
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso