ruby_debug_log: suppress warning

Old gcc (< 5 maybe?) warns that this variable is not initialized:

    debug.c: In function 'ruby_debug_log':
    debug.c:441:13: warning: 'r' may be used uninitialized in this function [-Wmaybe-uninitialized]
             if (r && len < MAX_DEBUG_LOG_MESSAGE_LEN) {
                 ^

I don't know if that is true, but adding "= 0" here must harm no one.

debug.c

@@ -380,7 +380,7 @@ ruby_debug_log(const char *file, int line, const char *func_name, const char *fm
 {
     char buff[MAX_DEBUG_LOG_MESSAGE_LEN] = {0};
     int len = 0;
-    int r;
+    int r = 0;
 
     // message title
     if (func_name && len < MAX_DEBUG_LOG_MESSAGE_LEN) {