asn1_find_node: added safety check on asn1_find_node()

This prevents a stack overflow in asn1_find_node() which
is triggered by too long variable names in the definitions
files. That means that applications have to deliberately
pass a too long 'name' constant to asn1_write_value()
and friends.  Reported by Jakub Jirasek.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>