gdb_server: fix invalid free

`gdb_service_free` calls `free(gdb_port_next)`, so this needs to be an
allocated string.  Otherwise we trip up detectors like Android's tagged
pointers.

Change-Id: Ib08ea55a38af4e15c4fbae95f10db0e3684ae1af
Signed-off-by: Jim Paris <jimparis@meta.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8768
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

src/server/gdb_server.c

@@ -3898,7 +3898,8 @@ static int gdb_target_add_one(struct target *target)
 				}
 			}
 		} else if (strcmp(gdb_port_next, "pipe") == 0) {
-			gdb_port_next = "disabled";
+			free(gdb_port_next);
+			gdb_port_next = strdup("disabled");
 		}
 	}
 	return retval;