MINOR: debug: report the number of entries in the backtrace

It's useful to get an indication of unresolved stuff or memory
corruption to have the apparent depth of the stack trace in the
output, especially if we dump nothing.

src/debug.c

@@ -111,7 +111,7 @@ void ha_thread_dump(struct buffer *buf, int thr, int calling_tid)
 		   would produce similar output to the following: */
 
 		if (nptrs)
-			chunk_appendf(buf, "             call trace:\n");
+			chunk_appendf(buf, "             call trace(%d):\n", nptrs);
 
 #ifndef USE_DL
 		/* if we can't rely on dladdr1() we won't figure what level is