The Ctrl-C handler in mysql closes the console while ReadConsole()
waits for console input.
But the main thread was detecting that ReadConsole() haven't read 
anything and was processing as if there're data in the buffer.
Fixed to handle correctly this error condition.
No test case added as the test relies on Ctrl-C sent to the client
from its console.


client/mysql.cc:
  Bug #29469: handle correctly console read error
mysys/my_conio.c:
  Bug #29469: 
   1. handle correctly console read error
   2. add boundry checks for console buffer.