[Ada] Fix obscure possible overflow in Uintp

[Arnaud Charlet <charlet at adacore dot com>]

In experimenting with building the compiler with overflow checks turned
on, we found a case where theoretically overflow might cause trouble if
not checked for compiling a-ngcefu.adb. Not clear if this could ever
really cause trouble with overflow checks off (the normal way the
compiler is compiled), and certainly not practical to find a small
test case for, but the original code was a bit junky anyway, and
this patch fixes the possible overflow

Tested on i686-pc-linux-gnu, committed on trunk

2008-07-30  Robert Dewar  <dewar@adacore.com>

	* uintp.adb (UI_GCD): Fix potential overflow

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