MP4: Fix Dead Store (Dead assignement/Dead increment) warning found by Clang > 3.3

Change-Id: I0befc8846b9af43e1563adb369ba4d332775ef76
Reviewed-on: https://code.wireshark.org/review/5778
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>