audioEnvelope.cpp: check for division by zero

audioEnvelope.cpp: check for division by zero

Fixes Coverity CID 709287: Division or modulo by zero (DIVIDE_BY_ZERO)
Call to "this->loadEnvelope()" set "this->m_envelopeMax" to 0.
182        loadEnvelope();
...
Division by expression "this->m_envelopeMax" which may be zero has undefined behavior
189        fy = m_envelope[x]/double(m_envelopeMax) * img.height();