Merge pull request #318 from igrr/bugfix/format_truncation_warning

coap_debug.c: fix format warning when compiling with GCC 8

src/coap_debug.c

@@ -291,7 +291,7 @@ msg_code_string(uint16_t c) {
   } else if (c >= 224 && c - 224 < (int)(sizeof(signals)/sizeof(const char *))) {
     return signals[c-224];
   } else {
-    snprintf(buf, sizeof(buf), "%u.%02u", c >> 5, c & 0x1f);
+    snprintf(buf, sizeof(buf), "%u.%02u", (c >> 5) & 0x7, c & 0x1f);
     return buf;
   }
 }