typo fixed


git-svn-id: svn://svn.code.sf.net/p/qwt/code/branches/qwt-6.1@1939 29e17f8d-cf37-471e-a288-72cc03a76eda
checking borderPath invokable before invoking to avoid Qt warnings


git-svn-id: svn://svn.code.sf.net/p/qwt/code/branches/qwt-6.1@1943 29e17f8d-cf37-471e-a288-72cc03a76eda