re PR debug/50869 (ice in vt_expand_var_loc_chain)

PR debug/50869
* cselib.c (cfa_base_preserved_regno): Initialize.
(cselib_expand_value_rtx_1): Don't expand it.
* var-tracking.c (vt_expand_var_loc_chain): Initialize depth.
Check it's only zero if result is NULL.

From-SVN: r180725