nmtui: handle write() errors correctly in nmt_newt_edit_string

It might happen that write() returns -1, but the errno is not EINTR.
In that case, the length would be incremented by 1, and the data pointer
to the data being written would be moved back by 1 byte on every error.

Make it so that the function exits with an error if it indicates an error.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1971

Fixes: 3bda3fb60c ('nmtui: initial import of nmtui')
(cherry picked from commit 13317bd536)

src/libnmt-newt/nmt-newt-utils.c

@@ -416,9 +416,18 @@ nmt_newt_edit_string(const char *data)
 
     len = data ? strlen(data) : 0;
     while (len) {
-        do
-            nwrote = write(fd, data, len);
-        while (nwrote == -1 && errno == EINTR);
+        nwrote = write(fd, data, len);
+
+        if (nwrote == -1) {
+            if (errno == EINTR) {
+                continue;
+            }
+
+            nmt_newt_message_dialog(_("Could not write to temporary file: %s"),
+                                    nm_strerror_native(errno));
+            nm_close(fd);
+            goto done;
+        }
 
         len -= nwrote;
         data += nwrote;