Nothing to free there.

pcapng_read_block() never sets *err_info if it returns
PCAPNG_BLOCK_NOT_SHB - that happens on an EOF, a short read, or on a
successful read of something that doesn't look like an SHB.

Change-Id: I23ad6aa1c95d800b068a798a4aad1d70d07ac281
Reviewed-on: https://code.wireshark.org/review/5686
Reviewed-by: Guy Harris <guy@alum.mit.edu>