ClientContext:_write_some: release buffer once data really accepted for sending

[d-a-v]

may fix #1872

[d-a-v]

I could trigger the bug This happens only with MSS!=1460 (with either lwip v1 or v2 - may be related with the 1460-sized buffers used in core, a PR for MSS instead will come later)

The tcp_write error is -1 (ERR_MEM) and is documented as follow:

The tcp_write() function will fail and return ERR_MEM if the length of the data exceeds the current send buffer size or if the length of the queue of outgoing segment is larger than the upper limit defined in lwipopts.h (TCP_SND_QUEUELEN). If the function returns ERR_MEM, the application should wait until some of the currently enqueued data has been successfully received by the other host and try again.