avcodec/exr: Fix undefined left shifts of negative numbers

Affected the FATE-tests exr-rgb-scanline-pxr24-half-uint32-13x9 and
exr-rgb-scanline-pxr24-uint32.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/exr.c

@@ -881,7 +881,7 @@ static int pxr24_uncompress(EXRContext *s, const uint8_t *src,
                 in     = ptr[3] + s->xdelta;
 
                 for (j = 0; j < s->xdelta; ++j) {
-                    uint32_t diff = (*(ptr[0]++) << 24) |
+                    uint32_t diff = ((uint32_t)*(ptr[0]++) << 24) |
                     (*(ptr[1]++) << 16) |
                     (*(ptr[2]++) << 8 ) |
                     (*(ptr[3]++));