PROXY: Fix warning-format-overflow directive argument is null
    
      CC       src/providers/proxy/libsss_proxy_la-proxy_id.lo
    In file included from src/util/util.h:47,
                     from src/providers/proxy/proxy.h:35,
                     from src/providers/proxy/proxy_id.c:30:
    In function ‘delete_user’,
        inlined from ‘get_pw_uid’ at src/providers/proxy/proxy_id.c:383:15,
        inlined from ‘proxy_account_info’ at src/providers/proxy/proxy_id.c:1617:19,
        inlined from ‘proxy_account_info_handler_send’ at src/providers/proxy/proxy_id.c:1760:20:
    src/util/debug.h:126:9: error: ‘%s’ directive argument is null
    [-Werror=format-overflow=]
      126 |         sss_debug_fn(__FILE__, __LINE__, __FUNCTION__, \
          |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      127 |                      __debug_macro_level, \
          |                      ~~~~~~~~~~~~~~~~~~~~~~
      128 |                      format, ##__VA_ARGS__); \
          |                      ~~~~~~~~~~~~~~~~~~~~~~
    src/providers/proxy/proxy_id.c:215:5: note: in expansion of macro ‘DEBUG’
      215 |     DEBUG(SSSDBG_TRACE_FUNC,
          |     ^~~~~
    src/providers/proxy/proxy_id.c: In function ‘proxy_account_info_handler_send’:
    src/providers/proxy/proxy_id.c:216:17: note: format string is defined here
      216 |           "User %s does not exist (or is invalid) on remote server,"
          |                 ^~
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>