Fixes bug introduced in 0101252e [1].
This is actually mainly a revert of that commit.

That commit introduced a bug in which a small value (under 1 octet) with
range spanning full 2 octets (65536) was encoded incorrectly as 1 octet.

Section 4 of X.691 defines 64K as 65536, and Section 11.5.7 explicitly states
"""
c) "range" is greater than 256 and less than or equal to 64K (the two-octet case).
"""

This was noted while encoding SBc-AP messaged generated with asn1c,
where a SEQUENCE OF uses aper_put_length() to set the number of items.

[1] https://github.com/mouse07410/asn1c/pull/91