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.

gdb/gdbserver/ChangeLog

@@ -1,3 +1,8 @@
+2013-09-02  Pedro Alves  <palves@redhat.com>
+
+	* server.c (gdb_read_memory): Return -1 on traceframe memory read
+	error instead of EIO.
+
 2013-08-28  Jan Kratochvil  <jan.kratochvil@redhat.com>
 
 	PR server/15604

gdb/gdbserver/server.c

@@ -716,7 +716,7 @@ gdb_read_memory (CORE_ADDR memaddr, unsigned char *myaddr, int len)
 
       if (traceframe_read_mem (current_traceframe,
 			       memaddr, myaddr, len, &nbytes))
-	return EIO;
+	return -1;
       /* Data read from trace buffer, we're done.  */
       if (nbytes > 0)
 	return nbytes;