Bug 1263870: Check allocation in WasmAstModule::declare; r=luke

MozReview-Commit-ID: bRq5jNR6uJ

--HG--
extra : rebase_source : e94a48b621c949a9dbf197afc6c21bdd61168d32
extra : histedit_source : 127c1de2fd8d8bcb19042d1a6c14e8cf16fa8633

js/src/asmjs/WasmTextToBinary.cpp

@@ -637,7 +637,9 @@ class WasmAstModule : public WasmAstNode
             return true;
         }
         *sigIndex = sigs_.length();
-        return sigs_.append(new (lifo_) WasmAstSig(WasmName(), Move(sig))) &&
+        auto* lifoSig = new (lifo_) WasmAstSig(WasmName(), Move(sig));
+        return lifoSig &&
+               sigs_.append(lifoSig) &&
                sigMap_.add(p, sigs_.back(), *sigIndex);
     }
     bool append(WasmAstSig* sig) {