Fix undefined behavior in `strncpy()` call

[LocalSpook]

strncpy()'s pointer parameters are marked restrict, but when file_browser_reload_directory_content() was being called to reload the directory that was currently open:

file_browser_reload_directory_content(browser, browser->directory)

it would try to copy its path buffer to itself, causing undefined behavior. The solution uses memmove(), which can handle overlapping pointers.

Found with Address Sanitizer.