libavformat/avio: Add avio_get_dyn_buf function

This commit adds the avio_get_dyn_buf function which allows accessing
the
content of a DynBuffer without destroying it.

This is required in matroskaenc for preliminary writing (correct) mkv
headers.

Context for this change is fixing regression bug #5977.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>