test: add coverage for utf8CheckIncomplete()

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

PR-URL: https://github.com/nodejs/node/pull/11419
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

test/parallel/test-string-decoder.js

@@ -83,6 +83,9 @@ 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');