tools: mkeficapsule: resource leak in read_bin_file()
Free the allocated buffer in case of an error.
Fixes: 9e63786e2b4b ("tools: mkeficapsule: rework the code a little bit")
Addresses-Coverity-ID: 345917 Resource leak
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>