TBB: replace assert() with runtime checks in PolarSSL module

Using assert() to check the length of keys and hashes included in
a certificate is not a safe approach because assert() only applies
to debug builds. A malformed certificate could exploit security
flaws in release binaries due to buffer overflows.

This patch replaces assert() with runtime checkings in the PolarSSL
authentication module, so malformed certificates can not cause a
memory overflow.

Change-Id: I42ba912020595752c806cbd242fe3c74077d993b