If grub_calloc() fails hist_lines becomes NULL. It means we loose the
reference to the previously allocated hist_lines and leak memory. With
this change on failure hist_lines still points to the old memory. So,
no leak, no state corruption.
Signed-off-by: Avnish Chouhan <avnish@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>