qemu-nbd: fix OpenBSD linker warning

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/qemu-nbd.c b/qemu-nbd.c
index 91ea014..6707ea5 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -353,7 +353,8 @@
         }
 
         if (socket == NULL) {
-            sprintf(sockpath, SOCKET_PATH, basename(device));
+            snprintf(sockpath, sizeof(sockpath), SOCKET_PATH,
+                     basename(device));
             socket = sockpath;
         }