Add xmemdupz() that duplicates a block of memory, and NUL terminates it.

A lot of places in git's code use code like:

  char *res;

  len = ... find length of an interesting segment in src ...;
  res = xmalloc(len + 1);
  memcpy(res, src, len);
  res[len] = '\0';
  return res;

A new function xmemdupz() captures the allocation, copy and NUL
termination.  Existing xstrndup() is reimplemented in terms of
this new function.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -211,19 +211,20 @@ static inline void *xmalloc(size_t size)
 	return ret;
 }
 
-static inline char *xstrndup(const char *str, size_t len)
+static inline void *xmemdupz(const void *data, size_t len)
 {
-	char *p;
-
-	p = memchr(str, '\0', len);
-	if (p)
-		len = p - str;
-	p = xmalloc(len + 1);
-	memcpy(p, str, len);
+	char *p = xmalloc(len + 1);
+	memcpy(p, data, len);
 	p[len] = '\0';
 	return p;
 }
 
+static inline char *xstrndup(const char *str, size_t len)
+{
+	char *p = memchr(str, '\0', len);
+	return xmemdupz(str, p ? p - str : len);
+}
+
 static inline void *xrealloc(void *ptr, size_t size)
 {
 	void *ret = realloc(ptr, size);