fix compilation and setting parameters to null

src/attributes/otherfaction.h

@@ -20,6 +20,7 @@ OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 extern "C" {
 #endif
 
+    struct unit;
     struct faction;
     struct attrib;
     extern struct attrib_type at_otherfaction;

src/kernel/config.c

@@ -1160,11 +1160,13 @@ void set_param(struct param **p, const char *key, const char *data)
         }
         p = &(*p)->next;
     }
-    par = malloc(sizeof(param));
-    par->name = _strdup(key);
-    par->data = _strdup(data);
-    par->next = *p;
-    *p = par;
+    if (data) {
+        par = malloc(sizeof(param));
+        par->name = _strdup(key);
+        par->data = _strdup(data);
+        par->next = *p;
+        *p = par;
+    }
 }
 
 void kernel_done(void)