Don't dereference NULL cs in cf_item_parse

Don't dereference NULL cs in cf_item_parse

Avoid dereferencing NULL cs in cf_item_parse and cf_reference_item it
invokes.

This fixes the following Coverity errors:

Error: FORWARD_NULL (CWE-476):
freeradius-server-2.2.6/src/main/conffile.c:900: var_compare_op: Comparing "cs" to null implies that "cs" might be null.
freeradius-server-2.2.6/src/main/conffile.c:932: var_deref_op: Dereferencing null pointer "cs".

Error: FORWARD_NULL (CWE-476):
freeradius-server-2.2.6/src/main/conffile.c:900: var_compare_op: Comparing "cs" to null implies that "cs" might be null.
freeradius-server-2.2.6/src/main/conffile.c:938: var_deref_op: Dereferencing null pointer "cs".

Error: FORWARD_NULL (CWE-476):
freeradius-server-2.2.6/src/main/conffile.c:958: var_compare_op: Comparing "cs" to null implies that "cs" might be null.
freeradius-server-2.2.6/src/main/conffile.c:963: var_deref_model: Passing null pointer "cs" to "cf_expand_variables", which dereferences it.
freeradius-server-2.2.6/src/main/conffile.c:782:4: deref_parm_in_call: Function "cf_reference_item" dereferences "outercs".
freeradius-server-2.2.6/src/main/conffile.c:597:25: var_assign_parm: Assigning: "cs" = "outercs".
freeradius-server-2.2.6/src/main/conffile.c:615:4: deref_var: Dereferencing "cs" (which is a copy of "outercs").

Error: FORWARD_NULL (CWE-476):
freeradius-server-2.2.6/src/main/conffile.c:958: var_compare_op: Comparing "cs" to null implies that "cs" might be null.
freeradius-server-2.2.6/src/main/conffile.c:973: var_deref_op: Dereferencing null pointer "cs".

Error: FORWARD_NULL (CWE-476):
freeradius-server-2.2.6/src/main/conffile.c:994: var_compare_op: Comparing "cs" to null implies that "cs" might be null.
freeradius-server-2.2.6/src/main/conffile.c:1009: var_deref_op: Dereferencing null pointer "cs".

Error: FORWARD_NULL (CWE-476):
freeradius-server-2.2.6/src/main/conffile.c:900: var_compare_op: Comparing "cs" to null implies that "cs" might be null.
freeradius-server-2.2.6/src/main/conffile.c:1041: var_deref_op: Dereferencing null pointer "cs".

Error: FORWARD_NULL (CWE-476):
freeradius-server-2.2.6/src/main/conffile.c:900: var_compare_op: Comparing "cs" to null implies that "cs" might be null.
freeradius-server-2.2.6/src/main/conffile.c:1051: var_deref_op: Dereferencing null pointer "cs".

Error: FORWARD_NULL (CWE-476):
freeradius-server-2.2.6/src/main/conffile.c:900: var_compare_op: Comparing "cs" to null implies that "cs" might be null.
freeradius-server-2.2.6/src/main/conffile.c:1054: var_deref_op: Dereferencing null pointer "cs".

Error: FORWARD_NULL (CWE-476):
freeradius-server-2.2.6/src/main/conffile.c:900: var_compare_op: Comparing "cs" to null implies that "cs" might be null.
freeradius-server-2.2.6/src/main/conffile.c:1066: var_deref_op: Dereferencing null pointer "cs".