fix overflow check

Author: lemire

src/UTF8.cs

@@ -1476,10 +1476,10 @@ private unsafe static (int utfadjust, int scalaradjust) calculateErrorPathadjust
                             contv += AdvSimd.CompareLessThanOrEqual(Vector128.AsSByte(currentBlock), largestcont);
                             n4v += AdvSimd.CompareGreaterThan(currentBlock, fourthByteMinusOne).AsSByte();
                             overflowCounter++;
-                            // We have a risk of overflow if overflowCounter reaches 255,
+                            // We have a risk of overflow if overflowCounter reaches 127,
                             // in which case, we empty contv and n4v, and update contbytes and
                             // n4.
-                            if (overflowCounter == 0xff)
+                            if (overflowCounter == 0x7f)
                             {
                                 overflowCounter = 0;
                                 contbytes += -AdvSimd.Arm64.AddAcrossWidening(contv).ToScalar();