fix(c-api) Fix how string's length is computed.

Obviously, it's just `feature.size`. Why did I do that…?

lib/c-api/src/wasm_c_api/unstable/target_lexicon.rs

@@ -300,7 +300,7 @@ pub unsafe extern "C" fn wasm_cpu_features_add(
     let feature = c_try!(
         str::from_utf8(slice::from_raw_parts(
             feature.data,
-            feature.size + 1,
+            feature.size,
         ));
         otherwise false
     );