Fix parsing crash when return is void

lib/parse.js

@@ -190,6 +190,7 @@ parse.SourceUnit = function(contract, expression) {
 parse.ReturnStatement = function(contract, expression) {
   register.statement(contract, expression);
 
+  expression.expression &&
   parse[expression.expression.type] &&
   parse[expression.expression.type](contract, expression.expression, true);
 };

test/sources/solidity/contracts/return/empty-return.sol

@@ -0,0 +1,7 @@
+pragma solidity ^0.5.0;
+
+contract Test {
+    function a(uint x) public pure {
+        return;
+    }
+}

test/units/expressions.js

@@ -16,4 +16,9 @@ describe('generic expressions / misc', () => {
     const info = util.instrumentAndCompile('return/return');
     util.report(info.solcOutput.errors);
   });
+
+  it('should compile after instrumenting function the returns void', () => {
+    const info = util.instrumentAndCompile('return/empty-return');
+    util.report(info.solcOutput.errors);
+  });
 });