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.h

@@ -176,6 +176,7 @@ char *strrep(const char *s, unsigned n);
 int split_pair(const char *s, const char *sep, char **l, char **r);
 
 int free_and_strdup(char **p, const char *s);
+int free_and_strndup(char **p, const char *s, size_t l);
 
 /* Normal memmem() requires haystack to be nonnull, which is annoying for zero-length buffers */
 static inline void *memmem_safe(const void *haystack, size_t haystacklen, const void *needle, size_t needlelen) {