tdf#147742 a11y: AccessibleGridControlTable needs SolarMutex

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

Change-Id: I4ab7a2626db09398c79fe53d85c9802136decbf7
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/130925
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
diff --git a/accessibility/source/extended/AccessibleGridControlTable.cxx b/accessibility/source/extended/AccessibleGridControlTable.cxx
index da63426..75a17f0 100644
--- a/accessibility/source/extended/AccessibleGridControlTable.cxx
+++ b/accessibility/source/extended/AccessibleGridControlTable.cxx
@@ -69,6 +69,8 @@ AccessibleGridControlTable::getAccessibleChild( sal_Int32 nChildIndex )

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

    ensureIsAlive();
    if(m_aTable.HasRowHeader() && m_aTable.HasColHeader())
        return 0;