Casting it to an int16_t was wrong and would preclude the check for high port
numbers (due to overflow).
This makes the type matching, but adds an explicit check to ensure the port
number passed in is within a valid range.
When setting up connectivity, e.g. with new network detection, the pref
wouldn't be checked (or honored), resulting in spurious startup calls
despite being preffed off. This fixes that.
Resolves#1474
Log spam fix. Suppresses 1,100 instances of NS_ENSURE_SUCCESS(rv, rv) failed with result 0x80520012 emitted from netwerk/base/nsIOService.cpp during linux64 debug testing. Backports [m-c 1333174](https://bugzilla.mozilla.org/show_bug.cgi?id=1333174).
