Add checking of size param and clarify usage in doc

[embhorn]

Description

The doc says that the size parameter of wc_SrpGetPublic is the size of the buffer passed in the pub parameter. This was not checked in the function. Also, the buffer was only cleared for the size of the public ephemeral value.

Fixes #3665

Testing

make check

Checklist

• [ ] added tests
• [x] updated/added doxygen
• [ ] updated appropriate READMEs
• [ ] Updated manual and documentation