Add CLEAR_CALLER_SAVED_REGS in leak-tree test

Add CLEAR_CALLER_SAVED_REGS in leak-tree test

Motivation for this change is similar to what has already been done in other
leak-* tests. That is, call CLEAR_CALLER_SAVED_REGS (currently used only on
PPC and MIPS arches) to clear temporary registers that might be holding
pointers lost in a previously called function.

This fixes memcheck/tests/leak-tree failure on some MIPS platforms.

Patch by Aleksandar Rikalo.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16173