Instead of adding a cast at the memory_error call, as needed for C++,
and have the reader understand the indirection, make it simple and
hardcode the generic memory error at the memory_error call site.
gdb/ChangeLog:
2015-11-17 Pedro Alves <palves@redhat.com>
* guile/scm-disasm.c (gdbscm_disasm_read_memory): Return -1 on
error instead of TARGET_XFER_E_IO.
(gdbscm_disasm_memory_error): Always pass TARGET_XFER_E_IO to
memory_error.