decompiler: fixed incorrect method references from single letter classes

test/org/jetbrains/java/decompiler/SingleClassesTest.java

@@ -80,6 +80,7 @@ public class SingleClassesTest {
   @Test public void testStringConcat() { doTest("pkg/TestStringConcat"); }
   @Test public void testJava9StringConcat() { doTest("java9/TestJava9StringConcat"); }
   @Test public void testMethodReferenceSameName() { doTest("pkg/TestMethodReferenceSameName"); }
+  @Test public void testMethodReferenceLetterClass() { doTest("pkg/TestMethodReferenceLetterClass"); }
 
   protected void doTest(String testFile, String... companionFiles) {
     ConsoleDecompiler decompiler = fixture.getDecompiler();