Fix uninitialised memory access in constant time functions

Fix an issue reported by Coverity whereby some constant time functions
called from the ssl decrypt code could potentially access uninitialised
memory.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

library/constant_time_internal.h

@@ -221,6 +221,13 @@ void mbedtls_ct_memcpy_if_eq( unsigned char *dest,
  * offset_secret, but only on \p offset_min, \p offset_max and \p len.
  * Functionally equivalent to `memcpy(dst, src + offset_secret, len)`.
  *
+ * \note                This function reads from \p dest, but the value that
+ *                      is read does not influence the result and this
+ *                      function's behavior is well-defined regardless of the
+ *                      contents of the buffers. This may result in false
+ *                      positives from static or dynamic analyzers, especially
+ *                      if \p dest is not initialized.
+ *
  * \param dest          The destination buffer. This must point to a writable
  *                      buffer of at least \p len bytes.
  * \param src           The base of the source buffer. This must point to a