sourcemaps: don't map a trailing generated-only segment

lib/internal/source_map/source_map.js

@@ -286,7 +286,8 @@ class SourceMap {
       }
 
       columnNumber += decodeVLQ(stringCharIterator);
-      if (isSeparator(stringCharIterator.peek())) {
+      if (!stringCharIterator.hasNext() ||
+          isSeparator(stringCharIterator.peek())) {
         ArrayPrototypePush(this.#mappings, [lineNumber, columnNumber]);
         continue;
       }
@@ -300,7 +301,8 @@ class SourceMap {
       sourceColumnNumber += decodeVLQ(stringCharIterator);
 
       let name;
-      if (!isSeparator(stringCharIterator.peek())) {
+      if (stringCharIterator.hasNext() &&
+          !isSeparator(stringCharIterator.peek())) {
         nameIndex += decodeVLQ(stringCharIterator);
         name = map.names?.[nameIndex];
       }

test/parallel/test-source-map-trailing-unmapped-segment.js

@@ -0,0 +1,67 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const { SourceMap } = require('node:module');
+
+// A segment with only a generated column marks the code after it as unmapped.
+// When it is the last segment of the mappings, it must not inherit the
+// original position of the segment before it.
+const unmapped = {
+  originalSource: undefined,
+  originalLine: undefined,
+  originalColumn: undefined,
+  name: undefined,
+};
+
+// Trailing unmapped segment on the same line as the last mapped segment.
+{
+  const sm = new SourceMap({
+    version: 3,
+    sources: ['a.js'],
+    names: [],
+    mappings: 'AAAA,K',
+  });
+
+  assert.deepStrictEqual(sm.findEntry(0, 5), {
+    generatedLine: 0,
+    generatedColumn: 5,
+    ...unmapped,
+  });
+  assert.deepStrictEqual(sm.findOrigin(1, 6), {});
+}
+
+// Trailing unmapped segment on a later line.
+{
+  const sm = new SourceMap({
+    version: 3,
+    sources: ['a.js'],
+    names: [],
+    mappings: 'AAAA;A',
+  });
+
+  assert.deepStrictEqual(sm.findEntry(1, 3), {
+    generatedLine: 1,
+    generatedColumn: 0,
+    ...unmapped,
+  });
+  assert.deepStrictEqual(sm.findOrigin(2, 4), {});
+}
+
+// A segment without a name field must not pick up a name from the names array.
+{
+  const sm = new SourceMap({
+    version: 3,
+    sources: ['a.js'],
+    names: ['should-not-be-used'],
+    mappings: 'AAAA',
+  });
+
+  assert.deepStrictEqual(sm.findEntry(0, 0), {
+    generatedLine: 0,
+    generatedColumn: 0,
+    originalSource: 'a.js',
+    originalLine: 0,
+    originalColumn: 0,
+    name: undefined,
+  });
+}