ExifImage: try to handle incorrect IFD format more gracefully

Test image provided in gomfunkel#14.

lib/exif/ExifImage.js

@@ -346,6 +346,9 @@ ExifImage.prototype.extractExifIFDEntries = function (data, offset, store, tags)
 {
   var entryCount = data.readUInt16(offset)
 
+  if (entryCount * 12 > data.length)
+	  throw new Error("Invalid number of IFD entries")
+
   for (var i = 0; i < entryCount; i++) {
     var start = offset + 2 + (i * 12)
     var entry = this.extractExifEntry(data, start, tags)
@@ -385,6 +388,9 @@ ExifImage.prototype.extractExifEntry = function (data, entryOffset, tags)
   }
 
   type = ExifImage.TIFFTYPES[field.type]
+  if (type == undefined)
+    return false
+
   type.size = type.len * type.count
 
   offset = entryOffset + 8