sfdisk: remove unnecessary size check [cppcheck]

sfdisk: remove unnecessary size check [cppcheck]

Following warning is false positive.  Size of the buffer is defined using
BUFSIZ, and so the strncpy() will never overwrite the last byte that is
initialized to zero in get_user_reply().

[disk-utils/sfdisk.c:137] -> [disk-utils/sfdisk.c:136]: (warning) Either the
condition 'bufsz!=0' is redundant or strncpy() argument nr 3 can have
invalid value.  The value is -1 but the valid values are '0:'.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>