tftp: on download, open local file only when first bit of data arrived

No reason to potentially clobber existing file before absolutely necessary.

function                                             old     new   delta
tftp_protocol                                       1947    2020     +73
tftp_main                                            393     376     -17
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 73/-17)             Total: 56 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed