network/dhcp: actually refuse to assign DHCP option when an invalid string is passed

Prompted by #30029.

src/network/networkd-dhcp-common.c

@@ -1014,9 +1014,11 @@ int config_parse_dhcp_send_option(
         }
         case DHCP_OPTION_DATA_STRING:
                 sz = cunescape(p, UNESCAPE_ACCEPT_NUL, &q);
-                if (sz < 0)
+                if (sz < 0) {
                         log_syntax(unit, LOG_WARNING, filename, line, sz,
                                    "Failed to decode DHCP option data, ignoring assignment: %s", p);
+                        return 0;
+                }
 
                 udata = q;
                 break;

test/fuzz/fuzz-network-parser/dhcp-option

@@ -0,0 +1,5 @@
+[DHCPv6]
+SendOption=1:string:\U
+SendVendorOption=123:1:string:\U
+[DHCPv4]
+SendOption=1:string:\U