Fix NULL objfile's section_offsets dereference during symbol load.

Fix NULL objfile's section_offsets dereference during symbol load.

gdb/ChangeLog:

        * symfile.c (syms_from_objfile_1): Renames syms_from_objfile.
        Remove call to init_entry_point_info.  Add OBJFILE's
        section_offsets and num_sections initialization.  Add note
        about entry info in the function documentation.
        (syms_from_objfile): New function.
        * coffread.c (coff_symfile_init): Remove call to
        init_entry_point_info.
        * machoread.c (macho_symfile_init): Likewise.
        * xcoffread.c(xcoff_symfile_init): Likewise.