avutil/buffer: use the default allocator if none is provided to av_buffer_pool_init2()

Signed-off-by: James Almer <jamrial@gmail.com>

doc/APIchanges

@@ -15,6 +15,9 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2020-06-xx - xxxxxxxxxx - lavu 56.50.100 - buffer.h
+  Passing NULL as alloc argument to av_buffer_pool_init2() is now allowed.
+
 2020-xx-xx - xxxxxxxxxx - lavc 58.88.100 - avcodec.h codec.h
   Move AVCodec-related public API to new header codec.h.
 

libavutil/buffer.c

@@ -229,6 +229,7 @@ AVBufferPool *av_buffer_pool_init2(int size, void *opaque,
     pool->size      = size;
     pool->opaque    = opaque;
     pool->alloc2    = alloc;
+    pool->alloc     = av_buffer_alloc; // fallback
     pool->pool_free = pool_free;
 
     atomic_init(&pool->refcount, 1);
@@ -310,6 +311,8 @@ static AVBufferRef *pool_alloc_buffer(AVBufferPool *pool)
     BufferPoolEntry *buf;
     AVBufferRef     *ret;
 
+    av_assert0(pool->alloc || pool->alloc2);
+
     ret = pool->alloc2 ? pool->alloc2(pool->opaque, pool->size) :
                          pool->alloc(pool->size);
     if (!ret)

libavutil/version.h

@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  56
-#define LIBAVUTIL_VERSION_MINOR  49
+#define LIBAVUTIL_VERSION_MINOR  50
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \