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.

matrix.c

@@ -34,18 +34,12 @@
 
 #include "config.h"
 #include "matrix.h"
-#include <gc.h>
+#include "alloc.h"
 
 /* 
  * Macros from "fm.h".
  */
 
-#define New(type)       ((type*)GC_MALLOC(sizeof(type)))
-#define NewAtom(type)   ((type*)GC_MALLOC_ATOMIC(sizeof(type)))
-#define New_N(type,n)   ((type*)GC_MALLOC((n)*sizeof(type)))
-#define NewAtom_N(type,n)       ((type*)GC_MALLOC_ATOMIC((n)*sizeof(type)))
-#define Renew_N(type,ptr,n)   ((type*)GC_REALLOC((ptr),(n)*sizeof(type)))
-
 #define SWAPD(a,b) { double tmp = a; a = b; b = tmp; }
 #define SWAPI(a,b) { int tmp = a; a = b; b = tmp; }