iff decoder: initialise HAM line decoder with first palette entry

The palette is organised into 'hold' and 'modify' pairs, hence why
the first palette entry is pal[1].

Fixes ticket #1701.
Signed-off-by: Peter Ross <pross@xvid.org>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/iff.c

@@ -428,7 +428,7 @@ static void decodeplane32(uint32_t *dst, const uint8_t *buf, int buf_size, int p
 static void decode_ham_plane32(uint32_t *dst, const uint8_t  *buf,
                                const uint32_t *const pal, unsigned buf_size)
 {
-    uint32_t delta = 0;
+    uint32_t delta = pal[1]; /* first palette entry */
     do {
         uint32_t first, second;
         DECODE_HAM_PLANE32(0);