linux: fix error checking in uv__open_file

uv__open_cloexec returns either the fd or a libuv error, which is -errno
on Unix, not -1.

PR-URL: https://github.com/libuv/libuv/pull/760
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>

src/unix/core.c

@@ -433,7 +433,7 @@ FILE* uv__open_file(const char* path) {
   FILE* fp;
 
   fd = uv__open_cloexec(path, O_RDONLY);
-  if (fd == -1)
+  if (fd < 0)
     return NULL;
 
    fp = fdopen(fd, "r");