The function _fp_exptmod_nct() is using WOLFSSL_NO_MALLOC to guard
using stack allocation vs malloc. It's twin function _fp_exptmod_ct()
is using WOLFSSL_SMALL_STACK for this. This is causing inappropriate use
of malloc() in a small stack environment. The no-malloc case will also
be kept so static memory and no-malloc fix still works.
1. Change the guards for `#ifndef WOLFSSL_NO_MALLOC` in the function
`_fp_exptmod_nct()` to `#if defined(WOLFSSL_SMALL_STACK) &&
!defined(WOLFSSL_NO_MALLOC)`.
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