bootchart: fix a potential buffer overrun

bootchart: fix a potential buffer overrun

If the configured number of samples was close to MAXSAMPLES,
the samples buffer could be overrun:

- by 1, because of off-by-one in the condition (samples > arg_samples_len),
and
- by many in case of an overrun, because the number of samples to
  capture was increased, instead of being decreased.

Simplify things by converting to a normal for-loop.

In store.c: change buffer size from 4095 to 4096. 4095 is a strange
number.