Fix instrumentation error for virtual modifiers (#832)

Author: cgewecke

lib/parse.js

@@ -217,9 +217,11 @@ parse.Modifiers = function(contract, modifiers) {
 };
 
 parse.ModifierDefinition = function(contract, expression) {
-  register.functionDeclaration(contract, expression);
-  parse[expression.body.type] &&
-  parse[expression.body.type](contract, expression.body);
+  if (expression.body) {
+    register.functionDeclaration(contract, expression);
+    parse[expression.body.type] &&
+    parse[expression.body.type](contract, expression.body);
+  }
 };
 
 parse.NewExpression = function(contract, expression) {

test/integration/projects/solc-8/contracts/Abstract_solc8.sol

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: MIT
+pragma solidity >=0.8.0 <0.9.0;
+
+abstract contract ContractB {
+    modifier onlyOwner() virtual;
+}