Fix crash when using a space command (such as \,) inside of sized parentheses. Add a test for this case.

Supported by Sage Days 29

Author: mdboom

lib/matplotlib/mathtext.py

@@ -1802,6 +1802,9 @@ class Kern(Node):
     when its *width* denotes additional spacing in the vertical
     direction.
     """
+    height = 0
+    depth = 0
+
     def __init__(self, width):
         Node.__init__(self)
         self.width = width