Resolver: added missing memory allocation error handling.

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index dcdbe0c..d59d0c4 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -2144,6 +2144,10 @@
 
     dst = ngx_resolver_alloc(r, n * sizeof(in_addr_t));
 
+    if (dst == NULL) {
+        return dst;
+    }
+
     j = ngx_random() % n;
 
     if (j == 0) {