fix build error with gcc9.2

with gcc9.2, it will get error like this:

In file included from rb-file-helpers.c:51:
rb-file-helpers.c: In function 'rb_find_plugin_data_file':
rb-debug.h:41:23: error: '%s' directive argument is null
[-Werror=format-overflow=]
   41 | #define rb_debug(...) rb_debug_realf (__FUNCTION__, __FILE__,
__LINE__, TRUE, __VA_ARGS__)
      |
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
rb-file-helpers.c:278:2: note: in expansion of macro 'rb_debug'
  278 |  rb_debug ("found '%s' when searching for file '%s' for plugin
'%s'",
      |  ^~~~~~~~
rb-file-helpers.c:278:20: note: format string is defined here
  278 |  rb_debug ("found '%s' when searching for file '%s' for plugin
'%s'",
      |                    ^~
cc1: all warnings being treated as errors

Signed-off-by: Chunmei Xu xuchunmei@linux.alibaba.com