wininet: don't evaluate netconn_get_avail_data() twice in netconn_read()

[Dan Kegel]

Using netconn_get_avail_data() in min() is funky, don't do it.

min() is a macro which evaluates its right hand argument twice,
which is not only extra work, it also might return different values
on the two calls, leading to min() sometimes returning a
larger value than either of its 'two' inputs.

Also added an assertion that makes the problem obvious.

Thanks to Sami Aario for spotting the overrun.

Fixes http://bugs.winehq.org/show_bug.cgi?id=27090 and
probably http://bugs.winehq.org/show_bug.cgi?id=22856
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