allow NULL empty blocks in blk2bstr

bstring/bstrlib.c

@@ -273,7 +273,7 @@ blk2bstr(const void *blk, int len)
 {
 	bstring b;
 	int i;
-	if (blk == NULL || len < 0) {
+	if (len < 0 || (len > 0 && blk == NULL)) {
 		return NULL;
 	}
 	b = malloc(sizeof(struct tagbstring));

bstring/bstrlib.h

@@ -147,6 +147,9 @@ bfromcstrrangealloc(int minl, int maxl, const char *str);
  * Create a bstring whose contents are described by the contiguous buffer
  * pointing to by blk with a length of len bytes.
  *
+ * If len is zero, blk may be NULL. Otherwise, blk must point to at least len
+ * bytes.
+ *
  * Note that this function creates a copy of the data in blk, rather than
  * simply referencing it. Compare with the blk2tbstr macro. If an error
  * occurs NULL is returned.

tests/bstest.c

@@ -200,7 +200,7 @@ START_TEST(core_001)
 {
 	/* tests with NULL */
 	test1_0(NULL, 10, NULL);
-	test1_0(NULL, 0, NULL);
+	test1_0(NULL, 0, "");
 	test1_0(NULL, -1, NULL);
 	/* normal operation tests */
 	test1_0(SHORT_STRING, sizeof(SHORT_STRING) - 1, SHORT_STRING);