[Toybox] [PATCH] main.c: fix unknown command handling.

[enh]

Fix a crash (found when I accidentally had a symlink to a toybox that
didn't support the command in question), and add disambiguating quotes
to the error message.

Before:

  $ ./toybox unknown
  munmap_chunk(): invalid pointer
  Aborted

After:

  $ ./toybox unknown
  toybox: Unknown command 'unknown'
---
 main.c | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)
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