ofz#3454 Floating-point-exception

Change-Id: I2432bff8e1c102b83f0b7aff1303a03539ce0350
Reviewed-on: https://gerrit.libreoffice.org/42580Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

tools/source/generic/fract.cxx

@@ -189,7 +189,9 @@ namespace
         bool fail = false;
         fail |= o3tl::checked_multiply(i.numerator() / gcd1, num / gcd2, num);
         fail |= o3tl::checked_multiply(i.denominator() / gcd2, den / gcd1, den);
-        i.assign(num, den);
+
+        if (!fail)
+            i.assign(num, den);
 
         return fail;
     }