Fixed an error when previewing the first line with Markdown extension

[kasecato]

Fixes #43858

• VSCode Version: 1.20.1
• OS Version: macOS High Sierra 10.13.3

Steps to Reproduce:

1. Open markdown file
2. Preview markdown
3. Scroll to first line in preview or double click on the first line

Does this issue occur when all extensions are disabled?: Yes

Logs

Developer Tools

ERR Running the contributed command:'_markdown.revealLine' failed.: Error: Running the contributed command:'_markdown.revealLine' failed.
 at e._executeContributedCommand (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:617:423)
 at e.$executeContributedCommand (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:617:744)
 at e._doInvokeHandler (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:584:983)
 at e._invokeHandler (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:584:701)
 at e._receiveRequest (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:583:836)
 at e._receiveOneMessage (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:583:605)
 at /Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:582:671
 at /Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:585:523
 at /Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:87:459
 at e.fire (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:88:729)
/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/workbench.main.js:157 WARNING: Promise with no error callback:152

Log (Extension Host)

[error] Error: Illegal value for `line`
	at t._lineAt (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:487:34)
	at Object.lineAt (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:485:801)
	at vscode.window.visibleTextEditors.filter.forEach.editor (/Applications/Visual Studio Code.app/Contents/Resources/app/extensions/markdown/out/commands.js:151:42)
	at Array.forEach (native)
	at RevealLineCommand.execute (/Applications/Visual Studio Code.app/Contents/Resources/app/extensions/markdown/out/commands.js:148:14)
	at e._executeContributedCommand (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:617:327)
	at e.$executeContributedCommand (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:617:744)
	at e._doInvokeHandler (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:584:983)
	at e._invokeHandler (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:584:701)
	at e._receiveRequest (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:583:836) _markdown.revealLine

[nzec]

Seems good.