test: add coverage for utf8CheckIncomplete()

This commit adds code coverage to utf8CheckIncomplete(), when the
lastNeed is greater than buffer length.

PR-URL: #11419
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>

Author: sushi90

test/parallel/test-string-decoder.js

@@ -87,6 +87,9 @@ assert.strictEqual(decoder.write(Buffer.from('F1', 'hex')), '');
 assert.strictEqual(decoder.write(Buffer.from('41F2', 'hex')), '\ufffdA');
 assert.strictEqual(decoder.end(), '\ufffd');
 
+// Additional utf8Text test
+decoder = new StringDecoder('utf8');
+assert.strictEqual(decoder.text(Buffer.from([0x41]), 2), '');
 
 // Additional UTF-16LE surrogate pair tests
 decoder = new StringDecoder('utf16le');