internal/fuzztest: set cap to len on test byte slice

Change-Id: I807995997fe7addf005eb639b0d4843cfb58f52b
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/218018
Reviewed-by: Herbie Ong <herbie@google.com>

internal/fuzztest/fuzztest.go

@@ -38,6 +38,7 @@ func Test(t *testing.T, fuzz func(b []byte) int) {
 			if err != nil {
 				t.Fatal(err)
 			}
+			b = b[:len(b):len(b)] // set cap to len
 			fuzz(b)
 		})
 	}