Exception in new terminal links

[Tyriar]

Easy repro:

Split a terminal such that lines get wrapped in the terminal
Move the mouse quickly around the left terminal

terminalLinkHelpers.ts:31 Uncaught TypeError: Cannot read property 'getCell' of undefined
    at Object.convertLinkRangeToBuffer (terminalLinkHelpers.ts:31)
    at terminalWebLinkProvider.ts:45
    at Array.forEach (<anonymous>)
    at TerminalWebLinkProvider.provideLink (terminalWebLinkProvider.ts:44)
    at e:\GitHub\microsoft\vscode\node_modules\xterm\lib\xterm.js:1
    at Array.forEach (<anonymous>)
    at e._askForLink (e:\GitHub\microsoft\vscode\node_modules\xterm\lib\xterm.js:1)
    at e._onHover (e:\GitHub\microsoft\vscode\node_modules\xterm\lib\xterm.js:1)
    at e._onMouseMove (e:\GitHub\microsoft\vscode\node_modules\xterm\lib\xterm.js:1)