gnutls_session_get_data: fix possible buffer overflow

gnutls_session_get_data: fix possible buffer overflow

The test to avoid the buffer overflow was always false because
session_data_size was set at the wrong place. This problem has been introduced
by this commit:

|commit ad4ed44c65e753e6d3a00104c049dd81826ccbf3
|Author: Nikos Mavrogiannopoulos <nmav@gnutls.org>
|Date:   Mon Nov 7 22:24:48 2005 +0000
|
|    This is the initial commit in the 1.3 branch. Ported from the PSK branch:
|    * PSK ciphersuites have been added.
|    * The session resumption data are now system independent.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>