unix: check for NULL in uv_os_unsetenv for parameter name

Fixes segfault of unit test on musl (AlpineLinux).
Add a check for parameter like uv_os_setenv do.

PR-URL: https://github.com/libuv/libuv/pull/1409
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

src/unix/core.c

@@ -1292,6 +1292,9 @@ int uv_os_setenv(const char* name, const char* value) {
 
 
 int uv_os_unsetenv(const char* name) {
+  if (name == NULL)
+    return -EINVAL;
+
   if (unsetenv(name) != 0)
     return -errno;