Fix issues detected by static analysis tool

Fix issues detected by static analysis tool

Buffer overflow is possible by index 'len' at wayland_panel_agent_module.cpp:2757, because 'readlink' may return 512. Function call at wayland_panel_agent_module.cpp:2750.
Buffer overflow is possible by index 'len' at wayland_panel_agent_module.cpp:2875, because 'readlink' may return 512. Function call at wayland_panel_agent_module.cpp:2868.
Pointer 'context_scim' which was dereferenced at wayland_panel_agent_module.cpp:1343 is compared to NULL value at wayland_panel_agent_module.cpp:1384.

Change-Id: Ib3acaaac112e1ce9dfc717a316804def2cc83327