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. The buffer was only cleared for the size of the public ephemeral value.

Fixes #3665

Testing

make check

Checklist

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

[Copilot]

Pull request overview

This PR addresses issue #3665 by adding buffer size validation and improving documentation for the wc_SrpGetPublic function in the SRP implementation. The changes aim to ensure the size parameter is properly checked before using it, and the buffer is cleared for the full size provided by the caller.

• Adds explicit buffer size validation before extracting the public key
• Clears the output buffer using the caller-provided size instead of just the modulus size
• Updates documentation to clarify the IN/OUT nature of the size parameter and provide a corrected example

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
wolfcrypt/src/srp.c	Adds buffer clearing for full size and size validation check for the wc_SrpGetPublic function
doc/dox_comments/header_files/srp.h	Clarifies size parameter documentation and fixes example code initialization

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[embhorn]

Retest this please

Multiple Arduino test failures

+ exit 1
Primary install failed: curl=0, sh=. Falling back...
/home/runner/work/_temp/c93b99ad-f6b3-4f95-8086-1712242c5803.sh: line 35: pushd: ./Arduino/sketches: No such file or directory
Error: Process completed with exit code 1.