Bug in av_store() Is this the right fix

In debugging a situation I discovered that the code called av_store() 
with an element that was already stored in that position.  av_store 
decrements the reference count, destroying what's about to be stored.

I know next to nothing about this.  Is the attached patch the right way 
to fix it?

• Bug in av_store() Is this the right fix by Karl Williamson