strip: Verify symbol table is a real symbol table

strip: Verify symbol table is a real symbol table

We didn't check the symbol table referenced from the relocation table
was a real symbol table. This could cause a crash if that section
happened to be an SHT_NOBITS section without any data. Fix this by
adding an explicit check.

       * src/strip.c (INTERNAL_ERROR_MSG): New macro that takes a
       message string to display.
       (INTERNAL_ERROR): Use INTERNAL_ERROR_MSG with elf_errmsg (-1).
       (remove_debug_relocations): Check the sh_link referenced
       section is real and isn't a SHT_NOBITS section.

https://sourceware.org/bugzilla/show_bug.cgi?id=32673

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