stack: don't try to parse non-truthy lines

Encountered while debugging some really oddball istanbul wrapping stuff.
This turned out to not be the root cause of the problem, but defense in
depth is usually a good idea.

lib/stack.js

@@ -173,7 +173,7 @@ var re = new RegExp(
 )
 
 function parseLine (line) {
-  var match = line.match(re)
+  var match = line && line.match(re)
   if (!match)
     return null