Fix double free.

[Quipyowert2]

When a command starting with an asterisk was encountered, ModuleConfig
was called, which calls AddToModList which frees its argument
sometimes. Then __execute_function tries to free the same pointer
again. This commit fixes this by only freeing rline in AddToModList if
it points at xasprintf'd memory, as freeing the memory from xasprintf
won't invalidate expaction.

Fixes #425.

• What does this PR do?
  Fixes a double free by conditionally freeing rline in AddToModList only if it doesn't point at tline anymore (by calling xasprintf).

The command causing the segfault was:
*FvwmIconMan*action Mouse 1 N sendcommand "Iconify off", sendcommand "FlipFocus", sendcommand "Raise"

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  • [https://raw.githubusercontent.com/fvwmorg/fvwm3/master/doc/README]Documentation updated (where appropriate)

  • Style guide followed (try and match the surrounding code where possible)

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• Issue number(s)

If this PR addresses any issues, please ensure the appropriate commit
message(s) contains:

Fixes #XXX

at the end of your commit message, where XXX should be replaced with the
relevant issue number.

If there is more than one issue fixed then use:

Fixes #XXX, fixes #YYY, fixes #ZZZ

[ThomasAdam]

Hi @Quipyowert2

While this is one approach, I really don't like littering the code with boolean logic as this just highlights a problem elsewhere to me -- namely that in this case, rline is a special case.

We can easily avoid using xasprintf() here if this stops this bool madness, a la strlcpy() for instance.

[Quipyowert2]

I removed the boolean variable and added a check of whether rline and tline are the same pointer. The boolean variable wasn't actually needed in this case.