Handle failed def lookup in action_data()

asm.js

@@ -1009,9 +1009,16 @@ export class Assembler {
 
     action_data(action) {
         const data = this.data.at(-1).data;
-        const value = action.value != null
-              ? action.value
-              : this.le(this.lookup_def(action.symbol), action.size);
+        let value = action.value;
+        if (value == undefined) {
+            const raw = this.lookup_def(action.symbol);
+            if (raw == undefined) {
+                console.error(
+                    `ERROR: Unable to resolve symbol ${action.symbol}`);
+                return;
+            }
+            value = this.le(raw, action.size);
+        }
         data.push(...value);
         this.pos.addr += action.size;
     }