APER encoding: Fix aper_put_length() with range=65536

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

skeletons/aper_support.c

@@ -22,7 +22,7 @@ aper_get_length(asn_per_data_t *pd, int range, int ebits, int *repeat) {
 
 	*repeat = 0;
 
-	if (range < 65536 && range >= 0)
+	if (range <= 65536 && range >= 0)
 		return aper_get_nsnnwn(pd, range);
 
 	if (aper_get_align(pd) < 0)
@@ -163,7 +163,7 @@ aper_put_length(asn_per_outp_t *po, int range, size_t length, int *need_eom) {
 	ASN_DEBUG("APER put length %zu with range %d", length, range);
 
 	/* 10.9 X.691 Note 2 */
-	if (range < 65536 && range >= 0)
+	if (range <= 65536 && range >= 0)
 		return aper_put_nsnnwn(po, range, length) ? -1 : (ssize_t)length;
 
 	if (aper_put_align(po) < 0)

tests/tests-skeletons/check-APER-support.c

@@ -76,8 +76,6 @@ check_round_trips_range65536() {
     check_round_trip(65536, 256);
     check_round_trip(65536, 65534);
     check_round_trip(65536, 65535);
-    check_round_trip(65536, 65536);
-    /* BUG: ^ this should fail but there's another unrelated bug, will be fixed in follow-up commit */
 }
 
 /*
@@ -103,6 +101,14 @@ check_encode_number_greater_than_range() {
     length = range;
     may_write = aper_put_length(&po, range, length, NULL);
     assert(may_write < 0);
+
+    /* Again value = range, with edge case 65536: */
+    memset(&po, 0, sizeof(po));
+    po.buffer = po.tmpspace;
+    po.nbits = 8 * sizeof(po.tmpspace);
+    length = range = 65536;
+    may_write = aper_put_length(&po, range, length, NULL);
+    assert(may_write < 0);
 }
 
 /*
@@ -123,7 +129,7 @@ check_range65536_encoded_as_2octet() {
     unsigned int bytes_needed = (po.buffer - po.tmpspace) + po.nboff/8;
     fprintf(stderr, "\naper_put_length(range=%d, len=%zu) => bytes_needed=%u\n",
             range, length, bytes_needed);
-    assert(bytes_needed == 1); /* BUG: should be 2 octets! */
+    assert(bytes_needed == 2);
 }
 
 int main() {