Fix TypeError: Cannot read property 'line' of undefined exception.

Triggered on any change when line 0 contains a CM bookmark.

I wrote the lazy `found.line ?` without cosidering what if line == 0.
Very embarrasing.  I should be fired :-(

In Mathdown this caused cben/mathdown#85
because Firepad renders other users' cursors with a CM bookmark
=> editing the first line immediately broke other windows editing same line.

Bug originated in 475901c on 2014-10-21

render-math.js

@@ -271,7 +271,7 @@ CodeMirror.hookMath = function(editor, MathJax) {
       // Verify it's in range, even after findMarks() - it returns
       // marks that touch the range, we want at least one char overlap.
       var found = mark.find();
-      if(found.line ?
+      if(found.line !== undefined ?
 	 /* bookmark */ posInsideRange(found, {from: from, to: to}) :
 	 /* marked range */ rangesOverlap(found, {from: from, to: to})) {
           mark.clear();