Fix a double-free issue in CFX_PathData::~CFX_PathData

In CPDFSDK_InterForm::SubmitFields, the buffer pointed by m_pBuffer is returned
and released by the caller. However, it will be released again in the destructor.

BUG=401580
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/481733002