Fix janet_formatbv() type when handling %d %u int specifiers

[zevv]

At https://github.com/janet-lang/janet/blob/master/src/core/pp.c#L849, janet_formatbv() pulls a long instead of an int when handling the %d specifier, potentially leading to wrong results:

janet_eprintf(">>> %d\n", (int32_t) - 1);

output:

>>> 4294967295

In the wild this shows like this:

(buffer/slice @"" 3)                                                  

error thrown:

error: start index 3 out of range [4294967295,0]

The same goes for %u and friends.