pict_type: add a value for unknown/none.

In commit bebe72f4a05d338e04ae9ca1e9c6b72749b488aa, 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 5129336714)

libavutil/avutil.h

@@ -112,7 +112,8 @@ enum AVMediaType {
 #define FF_API_OLD_IMAGE_NAMES (LIBAVUTIL_VERSION_MAJOR < 51)
 #endif
 enum AVPictureType {
-    AV_PICTURE_TYPE_I = 1, ///< Intra
+    AV_PICTURE_TYPE_NONE = 0, ///< Undefined
+    AV_PICTURE_TYPE_I,     ///< Intra
     AV_PICTURE_TYPE_P,     ///< Predicted
     AV_PICTURE_TYPE_B,     ///< Bi-dir predicted
     AV_PICTURE_TYPE_S,     ///< S(GMC)-VOP MPEG4