user - fix shadow file parsing on AIX (#62547)

Initialize variables in case the shadow file is not found.
Handle IndexErrors if something goes wrong with file parsing.

changelogs/fragments/user-aix-shadow-unbound-local.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - user - fix stack trace on AIX when attempting to parse shadow file that does not exist (https://github.com/ansible/ansible/issues/62510)

lib/ansible/modules/system/user.py

@@ -2501,12 +2501,15 @@ class AIX(User):
         """
 
         b_name = to_bytes(self.name)
+        b_passwd = b''
+        b_expires = b''
         if os.path.exists(self.SHADOWFILE) and os.access(self.SHADOWFILE, os.R_OK):
             with open(self.SHADOWFILE, 'rb') as bf:
                 b_lines = bf.readlines()
 
             b_passwd_line = b''
             b_expires_line = b''
+            try:
                 for index, b_line in enumerate(b_lines):
                     # Get password and lastupdate lines which come after the username
                     if b_line.startswith(b'%s:' % b_name):
@@ -2517,13 +2520,12 @@ class AIX(User):
                 # Sanity check the lines because sometimes both are not present
                 if b' = ' in b_passwd_line:
                     b_passwd = b_passwd_line.split(b' = ', 1)[-1].strip()
-            else:
-                b_passwd = b''
 
                 if b' = ' in b_expires_line:
                     b_expires = b_expires_line.split(b' = ', 1)[-1].strip()
-            else:
-                b_expires = b''
+
+            except IndexError:
+                self.module.fail_json(msg='Failed to parse shadow file %s' % self.SHADOWFILE)
 
         passwd = to_native(b_passwd)
         expires = to_native(b_expires) or -1