Fix possibly wrong execution of commands and out of boundary write with
unusual input. Correction and regression tests based on FreeBSD bin/95079.

While at it, fix another segmentation fault when us

Fix possibly wrong execution of commands and out of boundary write with
unusual input. Correction and regression tests based on FreeBSD bin/95079.

While at it, fix another segmentation fault when using ' ' as magic
character and also disallow '\0' as magic character. This cannot make any
sense and avoids a theoretical out of boundary read.

ok tb@

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