Attached is a patch for rxvt bug found by guarded malloc.
There is an underflow condition in rxvt_selection_adjust_kanji triggered
when one drags the mouse holding left button along the left side of the
window (c1 == 0). The added check prevents reading memory before the
allocated buffer. This seems to also be semantically correct as there is
no need to extend selection when there is nothing selected on the line.