sunos: preemptively fix uv_exepath(size=1)

I'm not 100% sure whether the behavior of readlink() with size=0 is
well-defined on Solaris; it can return either 0 or fail with EINVAL.
POSIX is not explicit about the mandated behavior.

This commit adds a little caution and makes it skip the readlink()
call when size=1 and simply writes the terminating zero byte.

PR-URL: https://github.com/libuv/libuv/pull/104
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

src/unix/sunos.c

@@ -304,7 +304,11 @@ int uv_exepath(char* buffer, size_t* size) {
     return -EINVAL;
 
   snprintf(buf, sizeof(buf), "/proc/%lu/path/a.out", (unsigned long) getpid());
-  res = readlink(buf, buffer, *size - 1);
+
+  res = *size - 1;
+  if (res > 0)
+    res = readlink(buf, buffer, res);
+
   if (res == -1)
     return -errno;