[C++] fix bounds checking on integer parsing (#4250)

* fix bounds checking on integer parsing

the previous code was allowing 255 for int8_t, similar for int16_t
and int32_t, and even negative values for unsignd types.

this patch fixes bounds checking for 8-bit, 16-bit and 32-bit types.
testing for both acceptable values and unacceptable values at the
boundaries are also improved.

bounds checking on 64-bit types isn't addressed by this patch.

* fix 'unary minus operator applied to unsigned type, result still unsigned'

* fix & placement