uthash.h: Swap multiplicands to put the widest ones first.

It was reported in #195 that Visual Studio 2019, on a 32-bit platform,
produces this warning:

    Warning C26451
    Arithmetic overflow: Using operator '*' on a 4 byte value
    and then casting the result to a 8 byte value. Cast the
    value to the wider type before calling operator '*' to
    avoid overflow (io.2).

I'm not 100% sure that this tweak will silence the warning,
but I think it might, and at least it's harmless.

Fixes #195.