fixes #15210 [backport:1.2] (#15237)

* fixes #15210 [backport:1.2]

* use patched version of bigints library

(cherry picked from commit ff13f8cc3c)

tests/misc/tunsignedconv.nim

@@ -75,3 +75,14 @@ let rangeVar = 0'u64 ..< limit
 doAssert repr(rangeVar) == """[a = 0,
 b = 0]
 """
+
+# bug #15210
+
+let a3 = not 0'u64
+var success = false
+try:
+  discard a3.int64
+except RangeDefect:
+  success = true
+
+doAssert success, "conversion should fail at runtime"