dup() can fail; check whether it does.

If, for example, you run out of file descriptors, dup() can fail, and
ws_dup() is a wrapper around it on UN*X.  Don't just pass the result of
ws_dup() to ws_fdopen(); instead, save its result, check against -1 and,
if it's -1, give up, otherwise pass it to ws_fdopen().

This addresses Coverity CID 1471708.

Also, if ws_fdopen() fails, close the descriptor we got from ws_dup();
this closes a possible FD leak.

ui/qt/endpoint_dialog.cpp

@@ -229,10 +229,17 @@ QUrl EndpointDialog::createMap(bool json_only)
         return QUrl();
     }
     // duplicate file descriptor as it is not allowed to perform a fclose before closing QFile
-    FILE* fp = ws_fdopen(ws_dup(fd), "wb");
+    int duped_fd = ws_dup(fd);
+    if (duped_fd == -1) {
+        QMessageBox::warning(this, tr("Map file error"), tr("Unable to create temporary file"));
+        g_free(hosts);
+        return QUrl();
+    }
+    FILE* fp = ws_fdopen(duped_fd, "wb");
     if (fp == NULL) {
         QMessageBox::warning(this, tr("Map file error"), tr("Unable to create temporary file"));
         g_free(hosts);
+        ws_close(duped_fd);
         return QUrl();
     }