Final polish

Drop the `copy` variable, we hava `avail <= nullFree`
so copy was just `avail`.

Author: hs-viktor

Data/ByteString/Builder/Internal.hs

@@ -919,13 +919,15 @@ cstringUtf8_step !ip !len !nullAt k (BufferRange op0 ope)
             len' = len - nread
         nullAt' <- c_strstr ip' (Ptr "\xc0\x80"#)
         cstringUtf8_step ip' len' nullAt' k (BufferRange op' ope)
-    | otherwise = do
-        let !copy = min avail nullFree
-        when (copy > 0) (S.memcpy op0 ip copy)
-        let !op' = op0 `plusPtr` copy
-            !ip' = ip `plusPtr` copy
-            len' = len - copy
+    | avail > 0 = do
+        -- avail <= nullFree
+        S.memcpy op0 ip avail
+        let !op' = op0 `plusPtr` avail
+            !ip' = ip `plusPtr` avail
+            len' = len - avail
         return $ bufferFull 1 op' (cstringUtf8_step ip' len' nullAt k)
+    | otherwise =
+        return $ bufferFull 1 op0 (cstringUtf8_step ip len nullAt k)
   where
     !avail = ope `minusPtr` op0
     !nullFree = nullAt `minusPtr` ip