gcoap: add asserts for path in gcoap_req_init

sys/include/net/gcoap.h

@@ -506,6 +506,9 @@ void gcoap_register_listener(gcoap_listener_t *listener);
  * @param[in] code      Request code: GCOAP_[GET|POST|PUT|DELETE]
  * @param[in] path      Resource path, *must* start with '/'
  *
+ * @pre @p path not `NULL`
+ * @pre @p path must start with `/`
+ *
  * @return  0 on success
  * @return  < 0 on error
  */

sys/net/application_layer/gcoap/gcoap.c

@@ -22,6 +22,7 @@
 #include <stdint.h>
 #include <stdatomic.h>
 
+#include "assert.h"
 #include "net/gcoap.h"
 #include "mutex.h"
 #include "random.h"
@@ -718,6 +719,8 @@ void gcoap_register_listener(gcoap_listener_t *listener)
 int gcoap_req_init(coap_pkt_t *pdu, uint8_t *buf, size_t len,
                    unsigned code, const char *path)
 {
+    assert((path != NULL) && (path[0] == '/'));
+
     (void)len;
 
     pdu->hdr = (coap_hdr_t *)buf;