In commit bebe72f4, the enum AV_PICTURE_TYPE_* was introduced. There are still places in the code where pict_type is used as an integer and there is a case where "pict_type = 0" with the explanation "let ffmpeg decide what to do". The new enum does not know a value of 0 and C++ will fail if compiling such programs anyway as it is refered as an int (and you cannot patch them properly).
(cherry picked from commit 51293367)