* common.cc (Sort_commons::operator()): Stabilize sort when both
entries are NULL.
(Symbol_table::do_allocate_commons_list): When allocating common
symbols, skip a symbol which is no longer common.
* symtab.h (Symbol::is_common): Test whether the symbol comes from
an object before checking its type.
* testsuite/common_test_2.c: New file.
* testsuite/common_test_3.c: New file.
* testsuite/Makefile.am (check_PROGRAMS): Add common_test_2.
(common_test_2_SOURCES, common_test_2_DEPENDENCIES): Define.
(common_test_2_LDFLAGS, common_test_2_LDADD): Define.
(common_test_2_pic.o, common_test_2.so): New targets.
(common_test_3_pic.o, common_test_3.so): New targets.
* testsuite/Makefile.in: Rebuild.
