paged_write backend functions iff the memory area in question has
a page_size != 0.
This is supposed to fix bug #19234: avrdude-5.3.1 segfaults when
stk500v1 tries to program an ATtiny15
git-svn-id: svn://svn.savannah.nongnu.org/avrdude/trunk@739 81a1dc3b-b13d-400b-aceb-764788c761c2