LUKS2 code read the whole header to buffer to verify checksum,
so malloc is called on unvalidated input size parameter.
This can cause out of memory or unintentional device reads.
(Header validation will fail later anyway - the size is unsupported.)
Just do not allow too small and too big allocations here and fail quickly.
Fixes: #683.
Test both primary and secondary header validation tests
with non-default LUKS2 json area size.
Check validation rejects config.keyslots_size with zero value.
Check validation rejects mismatching values for metadata size
set in binary header and in config json section.
