auth:gensec: Fully initialize struct spnego_data

auth:gensec: Fully initialize struct spnego_data

"Error: UNINIT (CWE-457):
samba-4.20.0rc2/auth/gensec/spnego.c:1129: var_decl: Declaring variable ""spnego_out"" without initializer.
samba-4.20.0rc2/auth/gensec/spnego.c:1154: uninit_use_in_call: Using uninitialized value ""spnego_out.negTokenTarg.negResult"" when calling ""spnego_write_data"".
 1152|   	}
 1153|
 1154|-> 	if (spnego_write_data(out_mem_ctx, out, &spnego_out) == -1) {
 1155|   		DEBUG(1, (""Failed to write SPNEGO reply to NEG_TOKEN_TARG\n""));
 1156|   		return NT_STATUS_INVALID_PARAMETER;"

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>