Return as early as possible in Scan_Integer
Return as early as possible in Scan_Integer

R3 on MacOS failed to parse "-0", because its strtoll handles empty
strings differently than Linux's

Here's result of strtoll("", NULL, 10):

platform,       return value,   errno
Linux,          0,              0
MacOS,          0,              EINVAL(22)

Avoid calling to strtoll with empty strings and return early solved this
problem.

hostilefork/rebol,draegtun/ren-c,rgchris/ren-c,draegtun/ren-c,giuliolunati/ren-c,draegtun/ren-c,rgchris/ren-c,codebybrett/ren-c,kealist/ren-c,giuliolunati/ren-c,draegtun/ren-c,hostilefork/rebol,kealist/ren-c,codebybrett/ren-c,codebybrett/ren-c,giuliolunati/ren-c,rgchris/ren-c,hostilefork/rebol,codebybrett/ren-c,kealist/ren-c,kealist/ren-c,codebybrett/ren-c,codebybrett/ren-c,draegtun/ren-c,giuliolunati/ren-c,hostilefork/rebol,kealist/ren-c,hostilefork/rebol,rgchris/ren-c,rgchris/ren-c,hostilefork/rebol,draegtun/ren-c,kealist/ren-c,giuliolunati/ren-c