fix: discard more bad source map positions (#3)

Restore code that was introduced in nyc 6.5.0 to handle bad source map locations. This fix was originally introduced into nyc's lib/source-map-cache.js, but when nyc transitioned to using istanbul-lib-source-maps this fix went astray.

see original PR: istanbuljs/nyc#255

lib/transformer.js

@@ -8,13 +8,25 @@ var pathutils = require('./pathutils'),
     libCoverage = require('istanbul-lib-coverage'),
     MappedCoverage = require('./mapped').MappedCoverage;
 
+function isInvalidPosition (pos) {
+    return !pos || !pos.line || !pos.column || pos.line < 0 || pos.column < 0;
+}
+
 /**
  * determines the original position for a given location
  * @param  {SourceMapConsumer} sourceMap the source map
  * @param  {Object} location the original location Object
  * @returns {Object} the remapped location Object
  */
 function getMapping(sourceMap, location, origFile) {
+
+    if (!location) {
+        return null;
+    }
+
+    if (isInvalidPosition(location.start) || isInvalidPosition(location.end)) {
+        return null;
+    }
 
     var start = sourceMap.originalPositionFor(location.start),
         end = sourceMap.originalPositionFor(location.end);