unix: fix uv_os_environ() null pointer check

Check the pointer to the allocated memory, not the pointer to the
pointer of the allocated memory. Previously, a failed allocation of
*envitems would lead to a NULL pointer dereference.

PR-URL: https://github.com/libuv/libuv/pull/2778
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Bartosz Sosnowski <bartosz@janeasystems.com>

src/unix/core.c

@@ -1258,7 +1258,7 @@ int uv_os_environ(uv_env_item_t** envitems, int* count) {
 
   *envitems = uv__calloc(i, sizeof(**envitems));
 
-  if (envitems == NULL)
+  if (*envitems == NULL)
     return UV_ENOMEM;
 
   for (j = 0, cnt = 0; j < i; j++) {