elfcompress: Handle shstrtab/symtab names more consistently

elfcompress: Handle shstrtab/symtab names more consistently

We track the shstrtab and symtab section names in case they need to be
renamed. These names are sometimes assigned a static string (from the
shstrtab string pool), sometimes created as a copy of an existing
string name and sometimes created by constructing a new string. This
means they might leak because they are never freed.

Fix this leak by defining the variables early (as NULL) before looping
over all sections, making sure to always assign a allocated (copy) of
the (new) name and explicitly free them at cleanup (run both on
success and failure).

This makes it easier to reason about allocation status and string
"ownership" making it easy to see there are no memory issues even in
the rare case of a .[z]debug section being used as a symtab or
shstrtab table (which is why there is no testcase, because it is
questionable whether such a file is actually valid).

	* src/elfcompress.c (process_file): Define shstrtab_name,
	shstrtab_newname, symtab_name and symtab_newname
	early. xstrdup shstrtab_name and symtab_name if they were
	assigned to a static string. Free all four strings at
	cleanup.

Signed-off-by: Mark Wielaard <mark@klomp.org>