virrandom: Avoid undefined behaviour in virRandomBits

If nbits is 64 (or greater) then shifting 1ULL left is undefined.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

src/util/virrandom.c

@@ -68,7 +68,9 @@ uint64_t virRandomBits(int nbits)
         return 0;
     }
 
+    if (nbits < 64)
         ret &= (1ULL << nbits) - 1;
+
     return ret;
 }