C example fixup (#2333)

* udp_example: add missing include

* udp_example: return on error

Otherwise we set src_addr_set even though it's still unknown.

examples/c/udp_example.c

@@ -11,6 +11,7 @@
 #include <netinet/in.h>
 #include <sys/socket.h>
 #include <time.h>
+#include <sys/time.h>
 
 #include <mavlink/common/mavlink.h>
 
@@ -82,8 +83,9 @@ void receive_some(int socket_fd, struct sockaddr_in* src_addr, socklen_t* src_ad
 
     if (ret < 0) {
         printf("recvfrom error: %s\n", strerror(errno));
+        return;
     } else if (ret == 0) {
-        // peer has done an orderly shutdown
+        // timeout, try again later
         return;
     }