avcodec/qdrw: Fix null pointer dereference

The RGB555 PACKBITSRGN case tries to read a palette, if such
palette is actually stored then it accesses a null pointer.
All 16bit samples i could find use DIRECTBITSRGN.

Fixes: 2065/clusterfuzz-testcase-minimized-6298930457346048

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/qdrw.c

@@ -58,6 +58,8 @@ static int parse_palette(AVCodecContext *avctx, GetByteContext *gbc,
             bytestream2_skip(gbc, 6);
             continue;
         }
+        if (avctx->pix_fmt != AV_PIX_FMT_PAL8)
+            return AVERROR_INVALIDDATA;
         r = bytestream2_get_byte(gbc);
         bytestream2_skip(gbc, 1);
         g = bytestream2_get_byte(gbc);
@@ -378,7 +380,9 @@ static int decode_frame(AVCodecContext *avctx,
             if ((ret = ff_get_buffer(avctx, p, 0)) < 0)
                 return ret;
 
-            parse_palette(avctx, &gbc, (uint32_t *)p->data[1], colors);
+            ret = parse_palette(avctx, &gbc, (uint32_t *)p->data[1], colors);
+            if (ret < 0)
+                return ret;
             p->palette_has_changed = 1;
 
             /* jump to image data */