EM_FORMATRANGE will render the text if wParam is non-zero, and there
is no reason to actually render the text when just getting the number
of pages.

EM_FORMATRANGE will set rc.bottom within the FORMATRANGE struct to be the
bottom of the area actually printed to, so this must be reset between calls
to make sure that the page boundaries are consistent between the pages.

name_buf is allocated on the stack and node_buf by heap_alloc.
Also don't leak node_buf when leaving the function.