test mpBrowser1 if not NULL + reorder the disposeAndClear calls
(must be reverse order compared to the order of Create calls)

Cherry-picked from 9be6c22e

Change-Id: Ia5481430791d9ab73e63d49a0ef0d5e0a6016605
Reviewed-on: https://gerrit.libreoffice.org/15885Reviewed-by: Joren De Cuyper <jorendc@libreoffice.org>
Tested-by: Joren De Cuyper <jorendc@libreoffice.org>