Fixed bugs in vasprintf_append()

* fixed bug in vasprintf_append() causing the input string to stay unchanged
** alen was accidentally set to 0 leading to vsnprintf() always being called with 1 as second argument, so only the terminating 0 was "appended"
* fixed possible memleak in vasprintf_append() in case realloc fails
** if realloc fails we (may) loose track of the memory to which *strp points resulting in a memory leak. *strp should only be overwritten if realloc succeeds.