Yet another explicit cast of pointer difference to uint32_t.

Yet another explicit cast of pointer difference to uint32_t.

This time we deal with pointers in 'Ins' array (the length of array
equals the calculated size of regular expression, which is currently
of type uint32_t: larger regular expressions will surely crash re2c).

So we can guarantee that, if regexp was correctly compiled to instructions,
than the first pointer points to some instuction in the array and second
pointer points to the beginning of array, so this cast is safe.

If compilation was incorrect than re2c will fail anyway.

Fixes [-Wconversion] warning.