pybind: fix error handling on getxattr

The ctypes bindings returned empty string
instead of raising exception.  This was a bug,
because it made it impossible to detect the
difference between missing xattr and empty
xattr.

Signed-off-by: John Spray <john.spray@redhat.com>

src/pybind/cephfs/cephfs.pyx

@@ -705,12 +705,19 @@ cdef class LibCephFS(object):
         try:
             ret_buf = <char *>realloc_chk(ret_buf, ret_length)
             with nogil:
-                ret = ceph_getxattr(self.cluster, _path, _name, ret_buf, ret_length)
+                ret = ceph_getxattr(self.cluster, _path, _name, ret_buf,
+                                    ret_length)
+
+            if ret < 0:
+                raise make_ex(ret, "error in getxattr")
 
             if ret > ret_length:
                 ret_buf = <char *>realloc_chk(ret_buf, ret)
                 with nogil:
-                    ceph_getxattr(self.cluster, _path, _name, ret_buf, ret)
+                    ret = ceph_getxattr(self.cluster, _path, _name, ret_buf,
+                                        ret)
+                if ret < 0:
+                    raise make_ex(ret, "error in getxattr")
 
             return ret_buf[:ret]
         finally: