Buffer: fix infinite loop when filling from a zero-length typed array

Buffer: fix infinite loop when filling from a zero-length typed array

njs_buffer_fill_typed_array() advanced the destination by
njs_min(byte_length, end - to) each iteration.  When the fill source
typed array was empty, the step was always zero and the loop spun
forever, hanging the worker on attacker-controlled input.

Zero-fill the range for an empty source, matching njs_buffer_fill_string()
and the QuickJS qjs_buffer_fill().