Use heap allocation for CSR PEM conversion on small stack builds

[jackctj117]

This pull request improves memory handling in the PEM conversion process within the TPM2 CSR generation code, particularly for environments with constrained stack space. The changes ensure that temporary buffers are dynamically allocated when needed, reducing stack usage and preventing potential buffer overflows.

Memory management improvements for PEM conversion:

• In both CSR_MakeAndSign and CSR_MakeAndSign_Cb functions in src/tpm2_wrap.c, added conditional logic to allocate the temporary buffer on the heap using XMALLOC when WOLFTPM_SMALL_STACK is defined, and to free it with XFREE after use. This helps support devices with limited stack space. [1] [2]
• Ensured that the size used for copying and conversion is consistently handled by introducing a derSz variable, improving code clarity and correctness. [1] [2]

[Copilot]

Pull request overview

This PR optimizes memory usage in TPM2 CSR generation for resource-constrained environments by conditionally using heap allocation instead of stack allocation for temporary PEM conversion buffers.

Changes:

• Added conditional compilation logic to allocate PEM conversion buffers on the heap when WOLFTPM_SMALL_STACK is defined
• Introduced derSz variable to improve clarity when tracking DER size throughout the conversion process

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