file_exists(): fix DEBUGF()

- %s was missing the argument (function name)
- add %p to print the function argument
- also don't call strlen() to test string nullity (and strlen() can't return a negative result)

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@28123 a1c6a512-1295-4272-9138-f99709370657

firmware/common/filefuncs.c

@@ -61,9 +61,9 @@ bool file_exists(const char *file)
     int fd;
 
 #ifdef DEBUG
-    if (!file || strlen(file) <= 0)
+    if (!file || !*file)
     {
-        DEBUGF("%s(): Invalid parameter!\n");
+        DEBUGF("%s(%p): Invalid parameter!\n", __func__, file);
         return false;
     }
 #endif