c_read_operations.cc: fix resource leak

CID 1188154 (#2 of 2): Resource leak (RESOURCE_LEAK)
 overwrite_var: Overwriting "op" in "op = rados_create_read_op()" leaks
 the storage that "op" points to.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>