Change PK module preprocessor check on word size

There were preprocessor directives in pk.c and pk_wrap.c that cheked
whether the bit length of size_t was greater than that of unsigned int.
However, the check relied on the POLARSSL_HAVE_INT64 macro being
defined which is not directly related to size_t. This might result in
errors in some platforms. This change modifies the check to use the
macros SIZE_MAX and UINT_MAX instead making the code more robust.
2 files changed