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>
diff --git a/internal/fuzztest/fuzztest.go b/internal/fuzztest/fuzztest.go
index 9c066dc..2b54eb5 100644
--- a/internal/fuzztest/fuzztest.go
+++ b/internal/fuzztest/fuzztest.go
@@ -38,6 +38,7 @@
 			if err != nil {
 				t.Fatal(err)
 			}
+			b = b[:len(b):len(b)] // set cap to len
 			fuzz(b)
 		})
 	}