Fix small memory leak of 40 bytes in sh_main()

[JohnoKing]

Minimal reproducer for the memory leak:

$ arch/*/bin/ksh true
true: true: cannot execute [Exec format error]

=================================================================
==458768==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 40 byte(s) in 1 object(s) allocated from:
    #0 0x7f7bd5360181 in malloc (/usr/lib/libasan.so.8+0x160181) (BuildId: 5f5511943a2d1bbff3f628464c43ad8c6ed91d5b)
    #1 0x562dc27e013d in sh_malloc /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/init.c:234
    #2 0x562dc2a08033 in path_fullname /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/path.c:526
    #3 0x562dc273ae45 in sh_main /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/main.c:246
    #4 0x562dc27385a2 in main /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/pmain.c:41
    #5 0x7f7bd4627d0d  (/usr/lib/libc.so.6+0x27d0d) (BuildId: 5e3b85a3eaa72d02eaae3882b22410ebda6b5555)
    #6 0x7ffc5d4ab4e1  (<unknown module>)

src/cmd/ksh93/sh/init.c:

• Free sh.st.filename before allocation if memory was already allocated to it during shell initialization.