parse: Fix two compiler warnings

Avoid that gcc on NetBSD reports the following compiler warnings:

    CC parse.o
parse.c: In function 'get_mult_time':
parse.c:175: warning: array subscript has type 'char'
parse.c: In function '__get_mult_bytes':
parse.c:221: warning: array subscript has type 'char'

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>