return early if there is no is_entries buffer

If there is a compilation error, is_entries may not be allocated, but
ic_size could be greater than 0.  If we don't have a buffer to iterate
over, just return early.  Otherwise GC could segv

[Bug #19173]

iseq.c

@@ -126,6 +126,14 @@ remove_from_constant_cache(ID id, IC ic)
 static void
 iseq_clear_ic_references(const rb_iseq_t *iseq)
 {
+    // In some cases (when there is a compilation error), we end up with
+    // ic_size greater than 0, but no allocated is_entries buffer.
+    // If there's no is_entries buffer to loop through, return early.
+    // [Bug #19173]
+    if (!ISEQ_BODY(iseq)->is_entries) {
+        return;
+    }
+
     for (unsigned int ic_idx = 0; ic_idx < ISEQ_BODY(iseq)->ic_size; ic_idx++) {
         IC ic = &ISEQ_IS_IC_ENTRY(ISEQ_BODY(iseq), ic_idx);
 

test/ruby/test_iseq.rb

@@ -355,6 +355,13 @@ class TestISeq < Test::Unit::TestCase
     end
   end
 
+  # [Bug #19173]
+  def test_compile_error
+    assert_raise SyntaxError do
+      RubyVM::InstructionSequence.compile 'using Module.new; yield'
+    end
+  end
+
   def test_compile_file_error
     Tempfile.create(%w"test_iseq .rb") do |f|
       f.puts "end"