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.

alloc.h

@@ -0,0 +1,39 @@
+/*
+ * by Scarlett. public domain.
+ * replacements for w3m's allocation macros which add overflow
+ * detection and concentrate the macros in one file
+ */
+#ifndef W3_ALLOC_H
+#define W3_ALLOC_H
+#include <gc.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <limits.h>
+
+static inline size_t
+z_mult_no_oflow_(size_t n, size_t size)
+{
+	if (size != 0 && n > SIZE_MAX / size) {
+		fprintf(stderr,
+		    "w3m: overflow in malloc, %zu*%zu\n", n, size);
+		exit(1);
+	}
+	return n * size;
+}
+
+#define New(type) \
+	(GC_MALLOC(sizeof(type)))
+
+#define NewAtom(type) \
+	(GC_MALLOC_ATOMIC(sizeof(type)))
+
+#define New_N(type, n) \
+	(GC_MALLOC(z_mult_no_oflow_((n), sizeof(type))))
+
+#define NewAtom_N(type, n) \
+	(GC_MALLOC_ATOMIC(z_mult_no_oflow_((n), sizeof(type))))
+
+#define New_Reuse(type, ptr, n) \
+	(GC_REALLOC((ptr), z_mult_no_oflow_((n), sizeof(type))))
+
+#endif /* W3_ALLOC_H */