Add null pointer test to String destructor

[jrowberg]

A rare but possible null pointer dereference occurs when a String object is destroyed after either a failed initialization or intentional invalidation. Adding a simple if (buffer) test before freeing it avoids this failure case.

[matthijskooijman]

I do not think this is needed, libc defines free(null) as a noop. From the free(3) manpage:

   The free() function frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc(), or realloc().  Otherwise, or if  free(ptr)  has  already  been

   called before, undefined behavior occurs.  If ptr is NULL, no operation is performed.

I see this PR was already reverted by @lxrobotics, possibly for this reason?

[aentinger]

@matthijskooijman you're right, according to spec free does nothing if the provided argument is 0.