Avoid undefined line in fromStringWithSourceMap

Avoid calling substr on `undefined` in `fromStringWithSourceMap`.
This can happen when passing in a source map that refers to lines past
the end of the passed-in string -- caller error, but let's not die.
Fixes mozilla#247

Author: tromey

dist/source-map.debug.js

@@ -2895,7 +2895,7 @@ return /******/ (function(modules) { // webpackBootstrap
 	          // There is no new line in between.
 	          // Associate the code between "lastGeneratedColumn" and
 	          // "mapping.generatedColumn" with "lastMapping"
-	          var nextLine = remainingLines[remainingLinesIndex];
+	          var nextLine = remainingLines[remainingLinesIndex] || '';
 	          var code = nextLine.substr(0, mapping.generatedColumn -
 	                                        lastGeneratedColumn);
 	          remainingLines[remainingLinesIndex] = nextLine.substr(mapping.generatedColumn -
@@ -2915,7 +2915,7 @@ return /******/ (function(modules) { // webpackBootstrap
 	        lastGeneratedLine++;
 	      }
 	      if (lastGeneratedColumn < mapping.generatedColumn) {
-	        var nextLine = remainingLines[remainingLinesIndex];
+	        var nextLine = remainingLines[remainingLinesIndex] || '';
 	        node.add(nextLine.substr(0, mapping.generatedColumn));
 	        remainingLines[remainingLinesIndex] = nextLine.substr(mapping.generatedColumn);
 	        lastGeneratedColumn = mapping.generatedColumn;

dist/source-map.js

@@ -97,7 +97,7 @@ SourceNode.fromStringWithSourceMap =
           // There is no new line in between.
           // Associate the code between "lastGeneratedColumn" and
           // "mapping.generatedColumn" with "lastMapping"
-          var nextLine = remainingLines[remainingLinesIndex];
+          var nextLine = remainingLines[remainingLinesIndex] || '';
           var code = nextLine.substr(0, mapping.generatedColumn -
                                         lastGeneratedColumn);
           remainingLines[remainingLinesIndex] = nextLine.substr(mapping.generatedColumn -
@@ -117,7 +117,7 @@ SourceNode.fromStringWithSourceMap =
         lastGeneratedLine++;
       }
       if (lastGeneratedColumn < mapping.generatedColumn) {
-        var nextLine = remainingLines[remainingLinesIndex];
+        var nextLine = remainingLines[remainingLinesIndex] || '';
         node.add(nextLine.substr(0, mapping.generatedColumn));
         remainingLines[remainingLinesIndex] = nextLine.substr(mapping.generatedColumn);
         lastGeneratedColumn = mapping.generatedColumn;