fs: fixup negative length in fs.truncate

doc/api/fs.md

@@ -406,6 +406,8 @@ try {
 If the file previously was shorter than `len` bytes, it is extended, and the
 extended part is filled with null bytes (`'\0'`):
 
+If `len` is negative then `0` will be used.
+
 #### `filehandle.utimes(atime, mtime)`
 <!-- YAML
 added: v10.0.0
@@ -2295,6 +2297,8 @@ open('temp.txt', 'r+', (err, fd) => {
 If the file previously was shorter than `len` bytes, it is extended, and the
 extended part is filled with null bytes (`'\0'`):
 
+If `len` is negative then `0` will be used.
+
 ### `fs.futimes(fd, atime, mtime, callback)`
 <!-- YAML
 added: v0.4.2

lib/fs.js

@@ -814,6 +814,7 @@ function truncate(path, len, callback) {
   }
 
   validateInteger(len, 'len');
+  len = MathMax(0, len);
   callback = maybeCallback(callback);
   fs.open(path, 'r+', (er, fd) => {
     if (er) return callback(er);

test/parallel/test-fs-truncate.js

@@ -280,3 +280,19 @@ function testFtruncate(cb) {
     );
   });
 });
+
+{
+  const file1 = path.resolve(tmp, 'truncate-file-1.txt');
+  fs.writeFileSync(file1, 'Hi');
+  fs.truncateSync(file1, -1);  // Negative cooerced to 0, No error.
+  assert(fs.readFileSync(file1).equals(Buffer.alloc(0)));
+}
+
+{
+  const file1 = path.resolve(tmp, 'truncate-file-2.txt');
+  fs.writeFileSync(file1, 'Hi');
+  // Negative cooerced to 0, No error.
+  fs.truncate(file1, -1, common.mustSucceed(() => {
+    assert(fs.readFileSync(file1).equals(Buffer.alloc(0)));
+  }));
+}