Use a single, consistent representation for an empty minimal

Use a single, consistent representation for an empty minimal
symbol table in an objfile.
* objfiles.c (terminate_minimal_symbol_table): New function.
(allocate_objfile): Call it.
* objfiles.h (terminate_minimal_symbol_table): New declaration.
(ALL_MSYMBOLS): No need to test whether (objfile)->msymbols is
non-NULL.
* minsyms.c (lookup_minimal_symbol_by_pc_section): To see whether
objfile has minimal symbols, compare minimal_symbol_count to zero,
instead of comparing msymbols with NULL.
* objfiles.c (have_minimal_symbols): Same.
* solib-sunos.c (solib_add_common_symbols): Call
terminate_minimal_symbol_table.
* symfile.c (reread_symbols): Same.