Fix crash when input source map has no mappings and no sources

Fixes #5

index.js

@@ -69,7 +69,7 @@ Concat.prototype.add = function(filePath, content, sourceMap) {
         });
       }
     } else {
-      if (sourceMap && sourceMap.sources)
+      if (sourceMap && sourceMap.sources && sourceMap.sources.length > 0)
         filePath = sourceMap.sources[0];
       for (var i = 1; i <= lines; i++) {
         this._sourceMap.addMapping({

package.json

@@ -1,6 +1,6 @@
 {
   "name": "concat-with-sourcemaps",
-  "version": "0.1.5",
+  "version": "0.1.6",
   "description": "Concatenate file contents with a custom separator and generate a source map",
   "homepage": "http://github.com/floridoo/concat-with-sourcemaps",
   "repository": "git://github.com/floridoo/concat-with-sourcemaps.git",
@@ -22,9 +22,9 @@
   },
   "devDependencies": {
     "jshint": "^2.5.0",
-    "tape": "^2.12.3",
+    "tape": "^3.0.2",
     "argg": "0.0.1",
-    "istanbul": "^0.2.8",
+    "istanbul": "^0.3.2",
     "faucet": "0.0.1",
     "coveralls": "^2.10.0"
   }

test/index.js

@@ -295,3 +295,22 @@ testCase('should keep source in sources with empty mappings', {
     sourceMap: '{"version":3,"file":"out.js","sources":["testXXX","test2","test3"],"names":[],"mappings":"AAAA;ACAA;ACAA"}'
   }
 });
+
+testCase('should not crash with an input source map with no mappings', {
+  separator: '\n',
+  sourceMapping: true,
+  outFile: 'out.js',
+  input: [
+    {
+      content: 'AAA\nBBB\nCCC',
+      sourceMap: '{"version":3,"file":"intermediate.js","sources":[],"names":[],"mappings":""}',
+      fileName: 'intermediate.js'
+    },
+    { content: 'EEE' },
+    { content: 'FFF' }
+  ],
+  output: {
+    content: 'AAA\nBBB\nCCC\nEEE\nFFF',
+    sourceMap: '{"version":3,"file":"out.js","sources":["intermediate.js", "test2", "test3"],"names":[],"mappings":"AAAA;AACA;AACA;ACFA;ACAA"}'
+  }
+});