Do not crash on AST if parsing failed

libsolidity/interface/CompilerStack.cpp

@@ -503,6 +503,9 @@ Scanner const& CompilerStack::scanner(string const& _sourceName) const
 
 SourceUnit const& CompilerStack::ast(string const& _sourceName) const
 {
+	if (m_stackState < ParsingSuccessful)
+		BOOST_THROW_EXCEPTION(CompilerError() << errinfo_comment("Parsing was not successful."));
+
 	return *source(_sourceName).ast;
 }