prevent segfault caused by bitidx++

This patch modifies Adafruit_DHT_Driver/Adafruit_DHT.c so that the code
that increments `bitidx` without bound is protected by #ifdef DEBUG.
This prevents this code from generating a segfault if readDHT is called several
times (which will ultimately cause bitidx to increment beyond the end of the
bits array).

Resolves github issue #78.