Correct input length check for uin128 fuzzer

In an earlier version of the uint128 fuzz target, there was a byte
which incidated the specific operation to test. However, that was
dropped for the much-more-effective approach of simply testing all
operations in each fuzz iteration.

fuzz/fuzz_targets/uint128_fuzz.rs

@@ -27,7 +27,7 @@ fn do_test(data: &[u8]) {
         } }
     }
 
-    if data.len() != 16*2 + 1 { return; }
+    if data.len() != 16*2 { return; }
     let (a_native, a) = read_ints!(0);
 
     // Checks using only a: