Fix ectest to not check a case which isn't guaranteed to work, and which we don't use.

When I removed the special case for EC_ILOG(0) in commit
 06390d08, it broke ec_dec_uint() with _ft=1
 (which should encode the value 0 using 0 bits).
This feature was tested by ectest.c, but not actually used by libcelt.
An assert has been added to ec_dec_uint() to ensure that we don't try to use
 this feature by accident.
ec_enc_uint() was actually correct, but support for this feature has been
 removed and the assert put in its place.