WebCrypto: fix AES-CTR out-of-bounds write on empty input

WebCrypto: fix AES-CTR out-of-bounds write on empty input

Previously, the AES-CTR block count helper computed the number of
blocks as "1 + (dend - 1) / dsor".  This underflows for a zero-length
input, so njs_ceil_div(0, 16) and qjs_ceiling_div(0, 16) returned
2^28 instead of 0.

The fix is to use the zero-safe "(dend + dsor - 1) / dsor" form.

The njs engine code was introduced in 7b2b7612 (0.7.0) and the
QuickJS counterpart in 75ca26f8 (0.8.10).