Fails to decode messages which are base64 encoded with padding

[ctron]

Describe the bug

libolm fails to decode megolm messages with are base64 encoded and have padding. The error it returns is "BAD_SIGNATURE".

To Reproduce
Steps to reproduce the behavior:

1. Create a new inbound group session
2. Try to decode a message which is base64 and has padding

Expected behavior

Should work or at least return "INVALID_BASE64".

Additional context

Encountered while writing a reproducer as part of matrix-org/vodozemac#69

[uhoreg]

Note to self for later on: It looks like libolm doesn't handle padded base64 at all. Probably the easiest way to fix this would be add a new function that takes the input buffer and length, checks for padding, and returns a new length for the unpadded string. Then, whenever we base64-decode a string, we do something like input_length = base64_unpadded_length(input_buffer, input_length); before decoding.