stdio-common: Consistently use 'num_digits_len' in 'vfscanf'

Make the only place use 'num_digits_len' enumeration constant where 10
is referred literally for a digit index in i18n handling for decimal
integers.  No change in code produced.

Reviewed-by: Arjun Shankar <arjun@redhat.com>

stdio-common/vfscanf-internal.c

@@ -1737,7 +1737,7 @@ __vfscanf_internal (FILE *s, const char *format, va_list argptr,
 #endif
 			    }
 
-			  if (n < 10)
+			  if (n < num_digits_len)
 			    {
 			      /* Found it.  */
 			      from_level = level;