Introduce free_and_strndup and use it in bus-message.c

v2: fix error in free_and_strndup()

When the orignal and copied message were the same, but shorter than specified
length l, memory read past the end of the buffer would be performed. A test
case is included: a string that had an embedded NUL ("q\0") is used to replace
"q".

v3: Fix one more bug in free_and_strndup and add tests.

v4: Some style fixed based on review, one more use of free_and_replace, and
make the tests more comprehensive.

src/basic/string-util.c

@@ -1004,7 +1004,7 @@ int free_and_strdup(char **p, const char *s) {
 
         assert(p);
 
-        /* Replaces a string pointer with an strdup()ed new string,
+        /* Replaces a string pointer with a strdup()ed new string,
          * possibly freeing the old one. */
 
         if (streq_ptr(*p, s))
@@ -1023,6 +1023,32 @@ int free_and_strdup(char **p, const char *s) {
         return 1;
 }
 
+int free_and_strndup(char **p, const char *s, size_t l) {
+        char *t;
+
+        assert(p);
+        assert(s || l == 0);
+
+        /* Replaces a string pointer with a strndup()ed new string,
+         * freeing the old one. */
+
+        if (!*p && !s)
+                return 0;
+
+        if (*p && s && strneq(*p, s, l) && (l > strlen(*p) || (*p)[l] == '\0'))
+                return 0;
+
+        if (s) {
+                t = strndup(s, l);
+                if (!t)
+                        return -ENOMEM;
+        } else
+                t = NULL;
+
+        free_and_replace(*p, t);
+        return 1;
+}
+
 #if !HAVE_EXPLICIT_BZERO
 /*
  * Pointer to memset is volatile so that compiler must de-reference