'rtbTable' may be underflowed because 'proba' is used without
checking for a non-negative as index of rtbTable[].

Add check: proba >= 0

Cc: stable@vger.kernel.org # v5.10+
Fixes: e0c1b49f5b67 ("lib: zstd: Upgrade to latest upstream zstd version 1.4.10")
Signed-off-by: Ilya Krutskih <devsec@tpz.ru>
---
 lib/zstd/compress/fse_compress.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/lib/zstd/compress/fse_compress.c b/lib/zstd/compress/fse_compress.c
index 44a3c10becf2..6b83f8bc943a 100644
--- a/lib/zstd/compress/fse_compress.c
+++ b/lib/zstd/compress/fse_compress.c
@@ -492,9 +492,10 @@ size_t FSE_normalizeCount (short* normalizedCounter, unsigned tableLog,
                 stillToDistribute--;
             } else {
                 short proba = (short)((count[s]*step) >> scale);
-                if (proba<8) {
-                    U64 restToBeat = vStep * rtbTable[proba];
-                    proba += (count[s]*step) - ((U64)proba<<scale) > restToBeat;
+		if ((proba >= 0) && (proba < 8)) {
+			U64 restToBeat = vStep * rtbTable[proba];
+
+			proba += (count[s]*step) - ((U64)proba<<scale) > restToBeat;
                 }
                 if (proba > largestP) { largestP=proba; largest=s; }
                 normalizedCounter[s] = proba;
-- 
2.43.0