lib:param: Add missing return code check

lib:param: Add missing return code check

"Error: NEGATIVE_RETURNS (CWE-394):
samba-4.20.0rc2/lib/param/loadparm.c:1275: negative_return_fn: Function ""lpcfg_map_parameter(""printing"")"" returns a negative number.
samba-4.20.0rc2/lib/param/loadparm.c:1275: assign: Assigning: ""parm_num"" = ""lpcfg_map_parameter(""printing"")"".
samba-4.20.0rc2/lib/param/loadparm.c:1278: negative_returns: Using variable ""parm_num"" as an index to array ""parm_table"".
 1276|   	}
 1277|
 1278|-> 	if (!lp_set_enum_parm(&parm_table[parm_num], pszParmValue, (int*)ptr)) {
 1279|   		return false;
 1280|   	}"

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>