validate arguments to mustCall(), default value

Updates expected to use modern default syntax and also validate the value
and throw an error rather than silently overwriting invalid arguments.

Would prevent issues such as the one fixed in the first commit on
nodejs#9031

test/common.js

@@ -402,8 +402,10 @@ function runCallChecks(exitCode) {
 }
 
 
-exports.mustCall = function(fn, expected) {
-  if (typeof expected !== 'number') expected = 1;
+exports.mustCall = function(fn, expected = 1) {
+  if (typeof expected !== 'number' || expected < 0) {
+    throw new RangeError(`Invalid expected value: ${expected}`);
+  }
 
   var context = {
     expected: expected,

test/parallel/test-common.js

@@ -5,3 +5,12 @@ var assert = require('assert');
 common.globalCheck = false;
 global.gc = 42;  // Not a valid global unless --expose_gc is set.
 assert.deepStrictEqual(common.leakedGlobals(), ['gc']);
+
+
+assert.throws(function() {
+  common.mustCall(function() {}, 'foo')();
+}, /invalid expected value: foo/i);
+
+assert.throws(function() {
+  common.mustCall(function() {}, /foo/)();
+}, /invalid expected value: \/foo\//i);