OpenSSL 3.0.0: handle empty cadata consistently

[tiran]

BPO	43920
Nosy	@tiran
PRs	bpo-43920: Make load_verify_locations(cadata) error message consistent (GH-25554) #25554 [3.9] bpo-43920: Make load_verify_locations(cadata) error message consistent (GH-25554) (GH-25555) #25555 [3.8] bpo-43920: Make load_verify_locations(cadata) error message consistent (GH-25554) (GH-25556) #25556

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/tiran'
closed_at = <Date 2021-04-23.14:37:43.140>
created_at = <Date 2021-04-23.09:52:47.430>
labels = ['expert-SSL', 'type-bug', '3.8', '3.9', '3.10', '3.11']
title = 'OpenSSL 3.0.0: handle empty cadata consistently'
updated_at = <Date 2021-04-23.14:37:43.140>
user = 'https://github.com/tiran'

bugs.python.org fields:

activity = <Date 2021-04-23.14:37:43.140>
actor = 'christian.heimes'
assignee = 'christian.heimes'
closed = True
closed_date = <Date 2021-04-23.14:37:43.140>
closer = 'christian.heimes'
components = ['SSL']
creation = <Date 2021-04-23.09:52:47.430>
creator = 'christian.heimes'
dependencies = []
files = []
hgrepos = []
issue_num = 43920
keywords = ['patch']
message_count = 4.0
messages = ['391673', '391682', '391689', '391699']
nosy_count = 1.0
nosy_names = ['christian.heimes']
pr_nums = ['25554', '25555', '25556']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43920'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10', 'Python 3.11']

[tiran]

OpenSSL 3.0.0-alpha15 handles empty data input to d2i_X509_bio() and PEM_read_bio_X509() differently. This causes cadata test for invalid data to fail with inconsistent error message.

Let's handle 0 result case more consistent and raise an error message that is more understandable than "no start line" or "not enough data".