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

Author: nornagon

lib/source-map-consumer.js

@@ -301,7 +301,9 @@ class BasicSourceMapConsumer extends SourceMapConsumer {
     const aStr = this._mappings;
     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,