avcodec/jpeglsdec: Don't presume the context to contain a JLSState

avcodec/jpeglsdec: Don't presume the context to contain a JLSState

Before 9b3c46a081a9f01559082bf7a154fc6be1e06c18 every call to
ff_jpegls_decode_picture() allocated and freed a JLSState. This commit
instead put said structure into the context of the JPEG-LS decoder to
avoid said allocation. But said function can also be called from other
MJPEG-based decoders and their contexts doesn't contain said structure,
leading to segfaults. This commit fixes this: The JLSState is now
allocated on the first call to ff_jpegls_decode_picture() and stored in
the context.

Found-by: Michael Niedermayer <michael@niedermayer.cc>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>