Avoid calling malloc with a zero length argument.

Avoid calling malloc with a zero length argument.

ibuf_open() will return an error in this case while ibuf_dynamic() accepts
a 0 len argument and just initialized the buffer and length to zero.
A later ibuf_realloc() call will take care of allocating the buffer.

Additionally switch from malloc() to calloc() when allocating the buffer
this way the buffer is initalized and in ibuf_reserve() an addtional
memset() is used to make sure that the reserved data is zeroed.

OK tb@