Bug #8255 points out a double free error in libwscale/utils.c file.
The double free is because the pointer to cascaded_context of an
sw_context is not set to NULL after freeing it. When the sw_context
is later freed, sws_freeContext is called on the cascaded_context,
causing a double free.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>