cbs: Remove useless initializations

Up until now, a temporary variable was used and initialized every time a
value was read in CBS; if reading turned out to be successfull, this
value was overwritten (without having ever been looked at) with the
value read if reading was successfull; on failure the variable wasn't
touched either. Therefore these initializations can be and have been
removed.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit b71a0367a6)

libavcodec/cbs_jpeg.c

@@ -45,7 +45,7 @@
 #define FUNC(name) cbs_jpeg_read_ ## name
 
 #define xu(width, name, range_min, range_max, subs, ...) do { \
-        uint32_t value = range_min; \
+        uint32_t value; \
         CHECK(ff_cbs_read_unsigned(ctx, rw, width, #name, \
                                    SUBSCRIPTS(subs, __VA_ARGS__), \
                                    &value, range_min, range_max)); \