tshark: fix a memory leak about display filter configuration

If the variable `dfilter' always points to malloc-ed memory, it should
be easier to avoid any leaks.
Leak:
```
Direct leak of 46 byte(s) in 1 object(s) allocated from:
    #0 0x7fadf5a67bc8 in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:144
    #1 0x7fadd7ecbe98 in g_malloc (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x57e98)
    #2 0x5556272dbfd5 in main /home/ivan/svnrepos/wireshark/tshark.c:1594
    #3 0x7fadd71ed0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
```

1 files changed, 3 insertions, 2 deletions

diff --git a/tshark.c b/tshark.c
index e6d282546e..174028ec90 100644
--- a/tshark.c
+++ b/tshark.c
@@ -750,7 +750,7 @@ main(int argc, char *argv[])
   volatile int         in_file_type = WTAP_TYPE_AUTO;
   gchar               *volatile cf_name = NULL;
   gchar               *rfilter = NULL;
-  gchar               *dfilter = NULL;
+  gchar               *volatile dfilter = NULL;
   dfilter_t           *rfcode = NULL;
   dfilter_t           *dfcode = NULL;
   e_prefs             *prefs_p;
@@ -1457,7 +1457,7 @@ main(int argc, char *argv[])
       /* already processed; just ignore it now */
       break;
     case 'Y':
-      dfilter = optarg;
+      dfilter = g_strdup(optarg);
       break;
     case 'z':
       /* We won't call the init function for the stat this soon
@@ -2344,6 +2344,7 @@ clean_exit:
   wtap_cleanup();
   free_progdirs();
   dfilter_free(dfcode);
+  g_free(dfilter);
   return exit_status;
 }