Fix crash with mal-formed config option.

Reproduction Steps. Both commands cause a segmentation fault:
```
dnsmasq --interface-name=,
dnsmasq --dynamic-host=,::,
```

Stack Trace (ASAN, v2.90)
```
==1817==ERROR: AddressSanitizer: SEGV on unknown address
0x000000000000 (pc 0x7f0203d22845 bp 0x7ffc3316b920 sp 0x7ffc3316b0a0
T0)
==1817==The signal is caused by a READ memory access.
==1817==Hint: address points to the zero page.
    #0 0x7f0203d22845 in __interceptor_strncmp
../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:488
    #1 0x5563361feeb3 in iface_allowed /opt/dnsmasq/src/network.c:361
    #2 0x556336203aca in iface_allowed_v6 /opt/dnsmasq/src/network.c:622
    #3 0x55633625cc4c in iface_enumerate /opt/dnsmasq/src/netlink.c:291
    #4 0x55633620573c in enumerate_interfaces /opt/dnsmasq/src/network.c:836
    #5 0x55633615c3fe in main /opt/dnsmasq/src/dnsmasq.c:367
```

The fault occurs due to a null-pointer dereference in iface_allowed()
at line 361:
```
if (strncmp(label, int_name->intr, IF_NAMESIZE) == 0)      //// here
int_name->intr is NULL
```

That occurs due to a null-pointer assignment in the option.c at line 4856:
```
new->intr = opt_string_alloc(arg);               //// if arg is empty
string, the opt_string_alloc will return NULL
```

src/option.c

@@ -4960,7 +4960,8 @@ err:
 	      arg = NULL; /* provoke error below */
 	  }
 	
-	if (!domain || !arg || !(new->name = canonicalise_opt(domain)))
+	if (!domain || !arg || !new->intr ||
+	    !(new->name = canonicalise_opt(domain)))
 	  ret_err(option == LOPT_DYNHOST ?
 		  _("bad dynamic host") : _("bad interface name"));