avcodec/jpeg2000dwt: Fix undefined shifts of negative numbers

Affected the vsynth*-jpeg2000 and the vsynth*-jpeg2000-97 FATE tests
(where * ranges over { 1, 2, 3, _lena }) as well as ticket #7983.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 5cf593adcd)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/jpeg2000dwt.c

@@ -255,7 +255,7 @@ static void dwt_encode97_int(DWTContext *s, int *t)
     line += 5;
 
     for (i = 0; i < w * h; i++)
-        t[i] <<= I_PRESHIFT;
+        t[i] *= 1 << I_PRESHIFT;
 
     for (lev = s->ndeclevels-1; lev >= 0; lev--){
         int lh = s->linelen[lev][0],