bugfix: ftp reads return empty

[tonygoldman]

Fixes #1928

Recent versions of PX4 and ArduCopter read payload->size bytes instead of kMaxDataLength.
Mavros was sending size = 0, causing all reads to return empty.

• Set req.header()->size to the actual bytes requested
• Guard the read loop against a redundant trailing request when read_size is an exact multiple of DATA_MAXSZ

[vooon]

LGTM, Thanks!