Fix a NULL pointer deref in ssl module. The cert parser did not handle CRL
distribution points with empty DP or URI correctly. A malicious or buggy
certificate can result into segfault.
Signed-off-by: Christian Heimes <christian@python.org>

https://bugs.python.org/issue35746
(cherry picked from commit a37f5243)
Co-authored-by: Christian Heimes <christian@python.org>