Fix isc_buffer_copyregion() for auto-reallocated buffers

Fix isc_buffer_copyregion() for auto-reallocated buffers

While isc_buffer_copyregion() calls isc_buffer_reserve() to ensure the
target buffer will have enough available space to append the contents of
the source region to it, the variables used for subsequently checking
available space are not updated accordingly after that call.  This
prevents isc_buffer_copyregion() from working as expected for
auto-reallocated buffers: ISC_R_NOSPACE will be returned if enough space
is not already available in the target buffer before it is reallocated.
Fix by calling isc_buffer_used() and isc_buffer_availablelength()
directly instead of assigning their return values to local variables.