Validate outputPath from compiler

The MemoryFs API throws an unclear error when using a relative path.

Author: SpaceK33z

middleware.js

@@ -55,6 +55,9 @@ module.exports = function(compiler, options) {
 	if(typeof options.reporter !== "function") options.reporter = defaultReporter;
 	if(typeof options.log !== "function") options.log = console.log.bind(console);
 	if(typeof options.warn !== "function") options.warn = console.warn.bind(console);
+	if(typeof compiler.outputPath === "string" && compiler.outputPath.lastIndexOf("/", 0) !== 0) {
+		throw new Error('`output.path` needs to be an absolute path or "/".');
+	}
 
 	// store our files in memory
 	var fs;

test/FileSystem.test.js

@@ -26,4 +26,12 @@ describe("FileSystem", function() {
 
 		should.strictEqual(firstFs, secondFs);
 	});
+
+	it("should throw on invalid outputPath config", function() {
+		var compiler = fakeWebpack();
+		compiler.outputPath = "./dist";
+		should.throws(function() {
+			middleware(compiler);
+		}, /output\.path/);
+	});
 });