Allow dest size of 0 in block decompress (python#27)

Author: jonathanunderwood

lz4/block/_block.c

@@ -253,7 +253,7 @@ decompress (PyObject * Py_UNUSED (self), PyObject * args, PyObject * kwargs)
       source_size -= hdr_size;
     }
 
-  if (dest_size <= 0 || dest_size > PY_SSIZE_T_MAX)
+  if (dest_size < 0 || dest_size > PY_SSIZE_T_MAX)
     {
       PyErr_Format (PyExc_ValueError, "Invalid size in header: 0x%zu",
                     dest_size);