Because libssh2_channel_read_ex can return 0.
"Note that a return value of zero (0) can in fact be a legitimate value and only signals that no payload data was read. It is not an error."
http://manpages.org/libssh2_channel_read_ex/3
/usr/local/Homebrew/Library/Homebrew/brew.rb:12:in `': Homebrew must be run under Ruby 2.3! You're running 2.0.0. (RuntimeError)
The command "brew install libssh2" failed and exited with 1 during
Because libssh2_channel_read_ex can return 0. "Note that a return value of zero (0) can in fact be a legitimate value and only signals that no payload data was read. It is not an error." http://manpages.org/libssh2_channel_read_ex/3