xlat: Always free head

xlat: Always free head

Move "head" deallocation into the path for handling fr_dhcp_decode_options
failure, in xlat_dhcp_options. This makes sure it is freed, as
fr_dhcp_decode_options is complicated and is not documented to guarantee
deallocation in case of error.

This fixes the following Coverity error:

Error: RESOURCE_LEAK (CWE-772):
freeradius-server-2.2.6/src/main/xlat.c:639: alloc_arg: "fr_dhcp_decode_options" allocates memory that is stored into "head".
freeradius-server-2.2.6/src/lib/dhcp.c:581:2: var_assign_parm: Assigning: "tail" = "head".
freeradius-server-2.2.6/src/lib/dhcp.c:647:4: alloc_fn: Storage is returned from allocation function "pairmake".
freeradius-server-2.2.6/src/lib/valuepair.c:1523:3: alloc_fn: Storage is returned from allocation function "pairmake_any".
freeradius-server-2.2.6/src/lib/valuepair.c:1406:2: alloc_fn: Storage is returned from allocation function "paircreate".
freeradius-server-2.2.6/src/lib/valuepair.c:174:2: alloc_fn: Storage is returned from allocation function "pairalloc".
freeradius-server-2.2.6/src/lib/valuepair.c:72:2: alloc_fn: Storage is returned from allocation function "malloc".
freeradius-server-2.2.6/src/lib/valuepair.c:72:2: var_assign: Assigning: "vp" = "malloc(312UL + name_len)".
freeradius-server-2.2.6/src/lib/valuepair.c:74:2: noescape: Resource "vp" is not freed or pointed-to in function "memset". [Note: The source code implementation of the function has been overridden by a builtin model.]
freeradius-server-2.2.6/src/lib/valuepair.c:134:2: return_alloc: Returning allocated memory "vp".
freeradius-server-2.2.6/src/lib/valuepair.c:174:2: var_assign: Assigning: "vp" = "pairalloc(da)".
freeradius-server-2.2.6/src/lib/valuepair.c:185:2: return_alloc: Returning allocated memory "vp".
freeradius-server-2.2.6/src/lib/valuepair.c:1406:2: var_assign: Assigning: "vp" = "paircreate(attr, 5)".
freeradius-server-2.2.6/src/lib/valuepair.c:1466:2: return_alloc: Returning allocated memory "vp".
freeradius-server-2.2.6/src/lib/valuepair.c:1523:3: return_alloc_fn: Directly returning storage allocated by "pairmake_any".
freeradius-server-2.2.6/src/lib/dhcp.c:647:4: var_assign: Assigning: "vp" = "pairmake(da->name, NULL, T_OP_ADD)".
freeradius-server-2.2.6/src/lib/dhcp.c:671:4: var_assign: Assigning: "*tail" = "vp".
freeradius-server-2.2.6/src/main/xlat.c:661: leaked_storage: Variable "head" going out of scope leaks the storage it points to.