Add xmallocz()

Add routine for allocating NUL-terminated memory block without risking
integer overflow in addition of +1 for NUL byte.

[jc: with suggestion from Bill Lear]
Signed-off-by: NIlari Liusvaara <ilari.liusvaara@elisanet.fi>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -343,6 +343,7 @@ extern void release_pack_memory(size_t, int);
 
 extern char *xstrdup(const char *str);
 extern void *xmalloc(size_t size);
+extern void *xmallocz(size_t size);
 extern void *xmemdupz(const void *data, size_t len);
 extern char *xstrndup(const char *str, size_t len);
 extern void *xrealloc(void *ptr, size_t size);

wrapper.c

@@ -34,6 +34,16 @@ void *xmalloc(size_t size)
 	return ret;
 }
 
+void *xmallocz(size_t size)
+{
+	void *ret;
+	if (size + 1 < size)
+		die("Data too large to fit into virtual memory space.");
+	ret = xmalloc(size + 1);
+	((char*)ret)[size] = 0;
+	return ret;
+}
+
 /*
  * xmemdupz() allocates (len + 1) bytes of memory, duplicates "len" bytes of
  * "data" to the allocated memory, zero terminates the allocated memory,
@@ -42,10 +52,7 @@ void *xmalloc(size_t size)
  */
 void *xmemdupz(const void *data, size_t len)
 {
-	char *p = xmalloc(len + 1);
-	memcpy(p, data, len);
-	p[len] = '\0';
-	return p;
+	return memcpy(xmallocz(len), data, len);
 }
 
 char *xstrndup(const char *str, size_t len)