avcodec/pixlet: Simplify pfx computation

Found by reviewing code related to CID1604365 Overflowed constant

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 0474614e6c)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pixlet.c

@@ -230,8 +230,8 @@ static int read_high_coeffs(AVCodecContext *avctx, const uint8_t *src, int16_t *
         if (cnt1 >= length) {
             cnt1 = get_bits(bc, nbits);
         } else {
-            pfx = 14 + ((((uint64_t)(value - 14)) >> 32) & (value - 14));
-            if (pfx < 1 || pfx > 25)
+            pfx = FFMIN(value, 14);
+            if (pfx < 1)
                 return AVERROR_INVALIDDATA;
             cnt1 *= (1 << pfx) - 1;
             shbits = show_bits(bc, pfx);