Prevent integer overflow when calculating buffer resizes

Make sure that we don't go haywire if an exponential buffer growth
operation winds up wrapping integer range.  Along the way, fix a
bookkeeping bug in BufferedTextOutput that would cause it to keep
spuriously realloc()ing on every append().

Bug 20674694

Change-Id: Ia845b7de36b90672a151a918ffc26c7da68e20a2