target: fix memory leak on multiple '-gdb-port' flag

In the odd case of multiple flags '-gdb-port' during 'target
create' or following 'configure', the new strdup()'ed value will
replace the old one without freeing it.

Free the old value (if it exists) before replacing it.

Change-Id: I1673346613ce7023880046e3a9ba473e75f18b8a
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6005
Tested-by: jenkins

src/target/target.c

@@ -5123,6 +5123,7 @@ no_params:
 				e = Jim_GetOpt_String(goi, &s, NULL);
 				if (e != JIM_OK)
 					return e;
+				free(target->gdb_port_override);
 				target->gdb_port_override = strdup(s);
 			} else {
 				if (goi->argc != 0)