tdf#147742 a11y: AccessibleGridControlHeader needs SolarMutex

... in `AccessibleGridControlHeader::getAccessibleIndexInParent`,
just like in the implementation of the other a11y UNO API methods.

Change-Id: Ib3bd44e73ae54bb1df05dc2714ce1f793f7222db
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/130924
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
diff --git a/accessibility/source/extended/AccessibleGridControlHeader.cxx b/accessibility/source/extended/AccessibleGridControlHeader.cxx
index 122e6ab..d88b342 100644
--- a/accessibility/source/extended/AccessibleGridControlHeader.cxx
+++ b/accessibility/source/extended/AccessibleGridControlHeader.cxx
@@ -74,6 +74,8 @@ AccessibleGridControlHeader::getAccessibleChild( sal_Int32 nChildIndex )

sal_Int32 SAL_CALL AccessibleGridControlHeader::getAccessibleIndexInParent()
{
    SolarMutexGuard aSolarGuard;

    ensureIsAlive();
    if(m_eObjType == vcl::table::TCTYPE_ROWHEADERBAR && m_aTable.HasColHeader())
        return 1;