Fix broken allocation logic in recently-rewritten jsonb_util.c.

Fix broken allocation logic in recently-rewritten jsonb_util.c.

reserveFromBuffer() failed to consider the possibility that it needs to
more-than-double the current buffer size.  Beyond that, it seems likely
that we'd someday need to worry about integer overflow of the buffer
length variable.  Rather than reinvent the logic that's already been
debugged in stringinfo.c, let's go back to using that logic.  We can
still have the same targeted API, but we'll rely on stringinfo.c to
manage reallocation.

Per report from Alexander Korotkov.