Prevent arithmetic overflow of 'i' in master.c:generate
requested to merge 3191-issue-45178-in-oss-fuzz-bind9-dns_master_load_fuzzer-integer-overflow-in-generate-v9_18 into v9_18
the value of 'i' in generate could overflow when adding 'step' to it in the 'for' loop. Use an unsigned int for 'i' which will give an additional bit and prevent the overflow. The inputs are both less than 2^31 and and the result will be less than 2^32-1.
(cherry picked from commit 5abdee90)
Closes #3191 (closed)