Added overflow checks for version numbers (ticket #762).

Added overflow checks for version numbers (ticket #762).

Both minor and major versions are now limited to 999 maximum.  In case of
r->http_minor, this limit is already implied by the code.  Major version,
r->http_major, in theory can be up to 65535 with current code, but such
values are very unlikely to become real (and, additionally, such values
are not allowed by RFC 7230), so the same test was used for r->http_major.