Resolver: fixed possible resource leak introduced in 5a16d40c63de.

Found by Coverity (CID 1351175).

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index d977475..abc23fc 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -1215,6 +1215,7 @@
 
             b->start = ngx_resolver_alloc(r, NGX_RESOLVER_TCP_RSIZE);
             if (b->start == NULL) {
+                ngx_resolver_free(r, b);
                 return NGX_ERROR;
             }
 
@@ -1236,6 +1237,7 @@
 
             b->start = ngx_resolver_alloc(r, NGX_RESOLVER_TCP_WSIZE);
             if (b->start == NULL) {
+                ngx_resolver_free(r, b);
                 return NGX_ERROR;
             }