[Bug #20234] Fix segv when parsing begin statement in method definition

In a method definition, the `begin` may not have an `nd_body`. When that
happens we get a null expr back from `last_expr_node` which causes a
segv for the following examples:

```ruby
def (begin;end).foo; end
def (begin;else;end).foo; end
def (begin;ensure;else;end).foo; end
```

In addition, I've added tests for other cases that weren't causing a
segv but appeared untested.`

Fixes https://bugs.ruby-lang.org/issues/20234

parse.y

@@ -1415,7 +1415,7 @@ last_expr_node(NODE *expr)
         if (nd_type_p(expr, NODE_BLOCK)) {
             expr = RNODE_BLOCK(RNODE_BLOCK(expr)->nd_end)->nd_head;
         }
-        else if (nd_type_p(expr, NODE_BEGIN)) {
+        else if (nd_type_p(expr, NODE_BEGIN) && RNODE_BEGIN(expr)->nd_body) {
             expr = RNODE_BEGIN(expr)->nd_body;
         }
         else {

test/ruby/test_parse.rb

@@ -1109,6 +1109,16 @@ def test_unused_variable
     assert_warning('') {o.instance_eval("def marg2((a)); nil; end")}
   end
 
+  def test_parsing_begin_statement_inside_method_definition
+    assert_equal :bug_20234, eval("def (begin;end).bug_20234; end")
+    assert_equal :bug_20234, eval("def (begin;rescue;end).bug_20234; end")
+    assert_equal :bug_20234, eval("def (begin;ensure;end).bug_20234; end")
+    assert_equal :bug_20234, eval("def (begin;rescue;else;end).bug_20234; end")
+
+    assert_raise(SyntaxError) { eval("def (begin;else;end).bug_20234; end") }
+    assert_raise(SyntaxError) { eval("def (begin;ensure;else;end).bug_20234; end") }
+  end
+
   def test_named_capture_conflict
     a = 1
     assert_warning('') {eval("a = 1; /(?<a>)/ =~ ''")}