Add overflow detection

Origin: http://marc.info/?l=openbsd-ports&m=142090828929750&w=2

* main.c: Call exit(1) when out of memory to avoid dereferencing null
pointers when gc's malloc fails.

* alloc.h: Replacements for w3m's allocation macros which add
overflow detection and concentrate the macros in one file.

* indep.h, libwc/charset.c, libwc/status.c, matrix.c: Use the
overflow-detecting allocation macros from alloc.h.

libwc/charset.c

@@ -1,8 +1,7 @@
 
 #include <stdlib.h>
 #include <ctype.h>
-#include <gc.h>
-#define New_N(type,n) ((type*)GC_MALLOC((n)*sizeof(type)))
+#include "../alloc.h"
 
 #include "wc.h"
 

libwc/status.c

@@ -1,7 +1,6 @@
 
 #include <string.h>
-#include <gc.h>
-#define New_N(type,n) ((type*)GC_MALLOC((n)*sizeof(type)))
+#include "../alloc.h"
 
 #include "wc.h"
 #ifdef USE_UNICODE