gh-99248: [Enum] fix negative number infinite loop (GH-99256)

[Enum] fix negative number infinite loop

- _iter_bits_lsb() now raises a ValueError if a negative number
  is passed in

- verify() now skips checking negative numbers for named flags
(cherry picked from commit 0b4ffb08cc)

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>

Lib/enum.py

@@ -114,9 +114,12 @@ def _make_class_unpicklable(obj):
         setattr(obj, '__module__', '<unknown>')
 
 def _iter_bits_lsb(num):
-    # num must be an integer
+    # num must be a positive integer
+    original = num
     if isinstance(num, Enum):
         num = num.value
+    if num < 0:
+        raise ValueError('%r is not a positive integer' % original)
     while num:
         b = num & (~num + 1)
         yield b
@@ -1856,6 +1859,9 @@ class verify:
                     if name in member_names:
                         # not an alias
                         continue
+                    if alias.value < 0:
+                        # negative numbers are not checked
+                        continue
                     values = list(_iter_bits_lsb(alias.value))
                     missed = [v for v in values if v not in member_values]
                     if missed: