mbedtls_ssl_read cannot be called with a NULL buffer even if len is 0,
as those are passed to memcpy and compilers doesn't like that.
Always pass a single byte (still len 0 so nothing is actually copied)
Fixes the following GCC 5 warnings and actual bugs:
```
drivers/unix/net_socket_posix.cpp:562:28: warning: comparison between 'enum IP::Type' and 'enum NetSocket::Type' [-Wenum-compare]
modules/gdscript/gdscript_function.cpp:792:26: warning: comparison of constant '17' with boolean expression is always true [-Wbool-compare]
modules/gdscript/gdscript_function.cpp:792:26: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
modules/gdscript/gdscript_parser.cpp:5082:58: warning: comparison of constant '6' with boolean expression is always false [-Wbool-compare]
modules/gdscript/gdscript_parser.cpp:5082:58: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
modules/mbedtls/stream_peer_mbed_tls.cpp:286:45: warning: comparison between 'enum StreamPeerTCP::Status' and 'enum StreamPeerSSL::Status' [-Wenum-compare]
modules/mbedtls/stream_peer_mbed_tls.cpp:313:45: warning: comparison between 'enum StreamPeerTCP::Status' and 'enum StreamPeerSSL::Status' [-Wenum-compare]
```
Note, it will only used by the Editor, not when running the game.
This allows package maintainer to compile Godot to use system installed
certificates when accessing the AssetLib.
This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
