lib/string.c: introduce memdup_nul() helper

This is completely analogous to the linux kernel's kmemdup_nul()
helper, apart from the lack of the gfp_t argument: Allocate a buffer
of size {len}+1, copy {len} bytes from the given buffer, and add a
final nul byte.

This pattern exists in a number of places, so this helper can reduce
some boilerplate code.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>

include/linux/string.h

@@ -143,6 +143,19 @@ void *memchr_inv(const void *, int, size_t);
  */
 void *memdup(const void *src, size_t len);
 
+/**
+ * memdup_nul() - allocate a buffer and copy in the contents, appending a nul byte
+ *
+ * Note that this returns a valid pointer even if @len is 0
+ *
+ * @src: data to copy in
+ * @len: number of bytes to copy
+ * Return: allocated buffer with the copied contents and an extra nul byte,
+ *      or NULL if not enough memory is available
+ *
+ */
+void *memdup_nul(const void *src, size_t len);
+
 unsigned long ustrtoul(const char *cp, char **endp, unsigned int base);
 unsigned long long ustrtoull(const char *cp, char **endp, unsigned int base);