gencode: Fix an undefined behavior in gen_mcode()

gencode: Fix an undefined behavior in gen_mcode()

Use a temporary 64-bit variable for the shift result.

Netmask length 0 is no longer a special case.

The error was:

$ testprogs/filtertest EN10MB net 192.168.0.0/16
gencode.c:7446:19: runtime error: left shift of 4294967295 by 16 places
  cannot be represented in type 'unsigned int'

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior gencode.c:7446:19

$ testprogs/filtertest EN10MB net 10.0.1.0/24
gencode.c:7446:19: runtime error: left shift of 4294967295 by 8 places
  cannot be represented in type 'unsigned int'

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior gencode.c:7446:19