extcap: Avoid double free of help.

Avoid double free of help when having multiple extcap interfaces.

Ping-Bug: 13218
Change-Id: I6a0d6afd645787f9814c02e84079b4e8763d05f1
Reviewed-on: https://code.wireshark.org/review/19261
Petri-Dish: Stig Bjørlykke <stig@bjorlykke.org>
Reviewed-by: Dario Lombardo <lomato@gmail.com>

extcap.c

@@ -385,7 +385,7 @@ static gboolean interfaces_cb(const gchar *extcap, const gchar *ifname _U_, gcha
         else if (int_iter->if_type == EXTCAP_SENTENCE_EXTCAP)
         {
             g_log(LOG_DOMAIN_CAPTURE, G_LOG_LEVEL_DEBUG, "  Extcap [%s] ", int_iter->call);
-            help = g_strdup(int_iter->help);
+            help = int_iter->help;
         }
 
         if (int_iter->if_type == EXTCAP_SENTENCE_INTERFACE)
@@ -403,7 +403,7 @@ static gboolean interfaces_cb(const gchar *extcap, const gchar *ifname _U_, gcha
             }
 
             int_iter->extcap_path = g_strdup(extcap);
-            int_iter->help = help;
+            int_iter->help = g_strdup(help);
             preserve_interface = extcap_if_add(int_iter);
         }