Fix #2767

Problem was the stack is empty if the local var declaration
is the first node of the compilation unit to be pushed.

pmd-java/etc/grammar/Java.jjt

@@ -1997,8 +1997,10 @@ void BlockStatement():
         |
         {
             List<Node> annotationsAndChildren = new ArrayList<Node>();
-            while (jjtree.peekNode() instanceof ASTAnnotation) {
-              annotationsAndChildren.add(jjtree.popNode());
+            if (jjtree.nodeArity() > 0) { // peekNode would throw if the stack is empty
+                while (jjtree.peekNode() instanceof ASTAnnotation) {
+                  annotationsAndChildren.add(jjtree.popNode());
+                }
             }
         }
         LocalVariableDeclaration()

pmd-java/src/test/java/net/sourceforge/pmd/lang/java/ast/ParserCornersTest.java

@@ -177,6 +177,16 @@ public class ParserCornersTest {
         java8.parseResource("GitHubBug207.java");
     }
 
+    @Test
+    public void testGitHubBug2767() {
+        // PMD fails to parse an initializer block.
+        // PMD 6.26.0 parses this code just fine.
+        java.withDefaultVersion("15-preview")
+            .parse("class Foo {\n"
+                       + "    {final int I;}\n"
+                       + "}\n");
+    }
+
     @Test
     public void testBug206() {
         java8.parse("public @interface Foo {" + "\n"