fix: convert result of allocate_mappings from signed to unsigned (mozilla#473)

Author: nornagon

lib/source-map-consumer.js

@@ -309,7 +309,9 @@ class BasicSourceMapConsumer extends SourceMapConsumer {
   _parseMappings(aStr, aSourceRoot) {
     const size = aStr.length;
 
-    const mappingsBufPtr = this._wasm.exports.allocate_mappings(size);
+    // Interpret signed result of allocate_mappings as unsigned, otherwise
+    // addresses higher than 2GB will be negative.
+    const mappingsBufPtr = this._wasm.exports.allocate_mappings(size) >>> 0;
     const mappingsBuf = new Uint8Array(this._wasm.exports.memory.buffer, mappingsBufPtr, size);
     for (let i = 0; i < size; i++) {
       mappingsBuf[i] = aStr.charCodeAt(i);