Fixed allocation failure detection in njs_backtrace_to_string().

Found by Coverity (CID 1472503).
diff --git a/src/njs_error.c b/src/njs_error.c
index 2b19c29..0fe92c2 100644
--- a/src/njs_error.c
+++ b/src/njs_error.c
@@ -1226,6 +1226,7 @@
 {
     size_t                 count;
     njs_chb_t              chain;
+    njs_int_t              ret;
     njs_uint_t             i;
     njs_backtrace_entry_t  *be, *prev;
 
@@ -1271,8 +1272,8 @@
         be++;
     }
 
-    njs_chb_join(&chain, dst);
+    ret = njs_chb_join(&chain, dst);
     njs_chb_destroy(&chain);
 
-    return NJS_OK;
+    return ret;
 }