Make Arena::free behave the same as free().
Make Arena::free behave the same as free().

If the given ptr is nullptr, then do nothing.
Fixes #1916

Qining/gapid,pmuetschard/gapid,pmuetschard/gapid,google/agi,google/agi,google/gapid,google/gapid,Qining/gapid,pmuetschard/gapid,Qining/gapid,google/gapid,google/agi,Qining/gapid,Qining/gapid,google/agi,google/gapid,google/agi,Qining/gapid,google/gapid,google/gapid,pmuetschard/gapid,pmuetschard/gapid,google/gapid,pmuetschard/gapid,google/agi,pmuetschard/gapid,google/agi,google/agi,Qining/gapid,pmuetschard/gapid,google/gapid,Qining/gapid