avcodec/vp56: Fix sign typo

Fixes: 664/clusterfuzz-testcase-4917047475568640

The change to fate is due to a truncated last frames which is now detected as damaged.

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

libavcodec/vp5.c

@@ -180,7 +180,7 @@ static int vp5_parse_coeff(VP56Context *s)
     int b, i, cg, idx, ctx, ctx_last;
     int pt = 0;    /* plane type (0 for Y, 1 for U or V) */
 
-    if (c->end >= c->buffer && c->bits >= 0) {
+    if (c->end <= c->buffer && c->bits >= 0) {
         av_log(s->avctx, AV_LOG_ERROR, "End of AC stream reached in vp5_parse_coeff\n");
         return AVERROR_INVALIDDATA;
     }

libavcodec/vp6.c

@@ -450,7 +450,7 @@ static int vp6_parse_coeff(VP56Context *s)
     int b, i, cg, idx, ctx;
     int pt = 0;    /* plane type (0 for Y, 1 for U or V) */
 
-    if (c->end >= c->buffer && c->bits >= 0) {
+    if (c->end <= c->buffer && c->bits >= 0) {
         av_log(s->avctx, AV_LOG_ERROR, "End of AC stream reached in vp6_parse_coeff\n");
         return AVERROR_INVALIDDATA;
     }

tests/ref/fate/vp5

@@ -249,4 +249,4 @@
 0,        243,        243,        1,   233472, 0x6f530ac6
 0,        244,        244,        1,   233472, 0x94f7466c
 0,        245,        245,        1,   233472, 0xa8c1d365
-0,        246,        246,        1,   233472, 0xedcff050
+0,        246,        246,        1,   233472, 0x8843293b