Fix broken gcov command

lib/codecov.js

@@ -556,6 +556,9 @@ var upload = function(args, on_success, on_failure) {
 }
 
 function sanitizeVar(arg) {
+  if (!arg) {
+    return ''
+  }
   return arg.replace(/&/g, '')
 }
 

test/index.test.js

@@ -282,4 +282,10 @@ describe('Codecov', function() {
       'real  run unsafe  command'
     )
   })
+
+  it('gracefully sanitizes undefined', function() {
+    expect(function() {
+      codecov.sanitizeVar(undefined)
+    }).not.toThrow()
+  })
 })