z80/disassembler: call memory_error_func when appropriate

If a call to the read_memory_func fails then we should call the
memory_error_func to notify the user of the disassembler of the
address that was a problem.

Without this GDB will report all memory errors as being at address
0x0.

opcodes/ChangeLog:

	* z80-dis.c (fetch_data): Call memory_error_func if the
	read_memory_func call fails.

opcodes/z80-dis.c

@@ -95,6 +95,8 @@ fetch_data (struct buffer *buf, disassemble_info * info, int n)
 			      n, info);
   if (r == 0)
     buf->n_fetch += n;
+  else
+    info->memory_error_func (r, buf->base + buf->n_fetch, info);
   return !r;
 }