LineReader: fix partial read

[ghost]

No description provided.

[xmrig]

Can you please provide test case for the fix.
Thank you.

[ghost]

xmrig stdout

[2020-10-13 18:01:49.960] [127.0.0.1:18888] received (61 bytes): "{"method":"ping","params":[]}                                "
[2020-10-13 18:01:49.961] [127.0.0.1:18888] received (61 bytes): "{"method":"ping","params":[]}                                "
[2020-10-13 18:01:49.963] [127.0.0.1:18888] received (61 bytes): "{"method":"ping","params":[]}                                "
[2020-10-13 18:01:49.965] [127.0.0.1:18888] received (123 bytes): "{"method"
[2020-10-13 18:01:49.965]  net      127.0.0.1:18888 JSON decode failed: "Missing a closing quotation mark in string."

sent packets

sent:  b'{"method":"ping","params":[]}                                \n'
sent:  b'{"method":"ping","params":[]}                                \n'
sent:  b'{"method":"ping","params":[]}                                '
sent:  b'\n{"method":"ping","params":[]} '
sent:  b'                               \n'

fixed xmrig stdout

[2020-10-13 18:09:44.986] [127.0.0.1:18888] received (61 bytes): "{"method":"ping","params":[]}                                "
[2020-10-13 18:09:44.987] [127.0.0.1:18888] received (61 bytes): "{"method":"ping","params":[]}                                "
[2020-10-13 18:09:44.990] [127.0.0.1:18888] received (61 bytes): "{"method":"ping","params":[]}                                "
[2020-10-13 18:09:44.991] [127.0.0.1:18888] received (61 bytes): "{"method":"ping","params":[]}                                "

[xmrig]

Thank you.