[Cython] Minor issue with switch conversion

[Jason Evans]

The switch conversion, as described at:

	http://wiki.cython.org/enhancements/switch

causes a compilation error for the following code:

===============
cdef int x = 42

if x == 1:
     print x
elif x == 2:
     print x
else:
     pass
===============

This is due to generating an empty default: clause.

Jason