bytestring: Test out_header_len != NULL before writing.

The documentation for |CBS_get_any_asn1_element| says that
|out_header_len| may be NULL, but in the case of an indefinite-length
element it would be written unconditionally.

Thanks to Doug Hogan for noticing this.

Change-Id: I17609b3465df73d42dd9efd75e783159aa99a59b
Reviewed-on: https://boringssl-review.googlesource.com/4993
Reviewed-by: Adam Langley <agl@google.com>