Do not output "undefined" if more mappings than lines.

lib/fromStringWithSourceMap.js

@@ -19,6 +19,7 @@ module.exports = function fromStringWithSourceMap(code, map) {
 	var currentSourceNodeLine;
 	mappings.forEach(function(mapping, idx) {
 		var line = lines[idx];
+		if(typeof line === 'undefined') return;
 		if(idx !== lines.length - 1) line += "\n";
 		if(!mapping)
 			return addCode(line);

test/fixtures/from-to-tests/mismatched-mappings.expected.map

@@ -0,0 +1,11 @@
+{
+  "version": 3,
+  "file": "mismatched-mappings.generated.txt",
+  "sources": [
+    "hello-world.txt"
+  ],
+  "sourcesContent": [
+    "function foobar() {\n  return; // foo\n}"
+  ],
+  "mappings": "AAAA;AACA;AADA"
+}

test/fixtures/from-to-tests/mismatched-mappings.generated.txt

@@ -0,0 +1,3 @@
+function foobar() {
+  return; // foo
+}

test/fixtures/from-to-tests/mismatched-mappings.input.map

@@ -0,0 +1,11 @@
+{
+  "version": 3,
+  "file": "mismatched-mappings.generated.txt",
+  "sources": [
+    "hello-world.txt"
+  ],
+  "sourcesContent": [
+    "function foobar() {\n  return; // foo\n}"
+  ],
+  "mappings": "AAAA,SAAS,MAAT,GAAmB;AACjB;CADF;AAAmB"
+}