Memory leak solved in MPEG4Extractor::parseChunk for cover art

Repeated leaks detected in mediaserver context. Most chunks small but
some bigger. Adding up it was not uncommon to see 200Mb footprint.
libc memdebug allocation call stack showed the root cause in "covr"
parsing - that explains why content often was empty but sometimes big.

Change-Id: I0de6c1eeef174d2529973ff9c6a020ec3dd44c75