server.c:gdb_read_memory: Fix error return.

server.c:gdb_read_memory: Fix error return.

When I added gdb_read_memory, with bits factored out from elsewhere, I
missed adjusting this error return.  gdb_read_memory has an interface
similar to Like GDB's xfer_partial:

> /* Read trace frame or inferior memory.  Returns the number of bytes
>   actually read, zero when no further transfer is possible, and -1 on
>   error.  Return of a positive value smaller than LEN does not
>   indicate there's no more to be read, only the end of the transfer.

Returning EIO, a positive value, is obviously bogus, for the caller
will confuse it with a successful partial transfer.

Found by inspection.

Tested on x86_64 Fedora 17.

gdb/gdbserver/
2013-09-02  Pedro Alves  <palves@redhat.com>

	* server.c (gdb_read_memory): Return -1 on traceframe memory read
	error instead of EIO.