Add a fuzzer to check CBS is a DER parser.

ECDSA_verify does a runtime check that our parser round-trips, but that
should already be true. Add a fuzzer to ensure it.

Change-Id: I396863b8f9ed66c6296cfb16f7197a63ae99e156
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/52733
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/fuzz/der_roundtrip_corpus/5bab61eb53176449e25c2c82f172b82cb13ffb9d b/fuzz/der_roundtrip_corpus/5bab61eb53176449e25c2c82f172b82cb13ffb9d
new file mode 100644
index 0000000..0d758c9
--- /dev/null
+++ b/fuzz/der_roundtrip_corpus/5bab61eb53176449e25c2c82f172b82cb13ffb9d
@@ -0,0 +1 @@
+?
\ No newline at end of file