Fix undefined error when debugging modules (DonJayamanne#589)

Fixes an issue where launching a debug session with modules results in the error "Cannot read property 'cwd' of undefined"

Author: xoob

src/client/debugger/Main.ts

@@ -188,7 +188,7 @@ export class PythonDebugger extends DebugSession {
  }
  else {
  // When using modules ensure the cwd has been provided
- if (typeof args.cwd !== 'string' || args.cwd.length === 0 || this.launchArgs.cwd === 'null') {
+ if (typeof args.cwd !== 'string' || args.cwd.length === 0 || (this.launchArgs && this.launchArgs.cwd === 'null')) {
  return this.sendErrorResponse(response, 2001, `'cwd' in 'launch.json' needs to point to the working directory`);
  }
  }
@@ -674,4 +674,4 @@ export class PythonDebugger extends DebugSession {
  }
 }
 
-DebugSession.run(PythonDebugger);
+DebugSession.run(PythonDebugger);