unix: fix potential bug if dup2 fails in uv_spawn
unix: fix potential bug if dup2 fails in uv_spawn

PR-URL: https://github.com/libuv/libuv/pull/309
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>

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