Check for arithmetic overflow when calculating sizes, to prevent

Check for arithmetic overflow when calculating sizes, to prevent
some buffer-overflow issues.  These patches are conservative, in the
sense that when I couldn't determine whether an overflow was possible,
I inserted a run-time check.
* regex_internal.h (re_xmalloc, re_xrealloc, re_x2realloc): New macros.
(SIZE_MAX) [!defined SIZE_MAX]: New macro.
(re_alloc_oversized, re_x2alloc_oversized, re_xnmalloc):
(re_xnrealloc, re_x2nrealloc): New inline functions.
* lib/regcomp.c (init_dfa, analyze, build_range_exp, parse_bracket_exp):
(build_equiv_class, build_charclass): Check for arithmetic overflow
in size expression calculations.
* lib/regex_internal.c (re_string_realloc_buffers):
(build_wcs_upper_buffer, re_node_set_add_intersect):
(re_node_set_init_union, re_node_set_insert, re_node_set_insert_last):
(re_dfa_add_node, register_state): Likewise.
* lib/regexec.c (re_search_stub, re_copy_regs, re_search_internal):
(prune_impossible_nodes, push_fail_stack, set_regs, check_arrival):
(build_trtable, extend_buffers, match_ctx_init, match_ctx_add_entry):
(match_ctx_add_subtop, match_ctx_add_sublast): Likewise.