smtp.c: Fixed compilation warnings

smtp.c:2357 warning: adding 'size_t' (aka 'unsigned long') to a string
            does not append to the string
smtp.c:2375 warning: adding 'size_t' (aka 'unsigned long') to a string
            does not append to the string
smtp.c:2386 warning: adding 'size_t' (aka 'unsigned long') to a string
            does not append to the string

Used array index notation instead.