fdt: Check if the FDT address is configured

In case the "fdt addr" command wasn't ran yet and any other "fdt"
subcommand was issued, the system crashed due to NULL pointer being
used.

This is caused by "fdt addr" command setting up a pointer to the
FDT memory location. Prior issuing "fdt addr", the pointer is NULL
so calling any other subcommands crashed the u-boot.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>

common/cmd_fdt.c

@@ -114,10 +114,21 @@ int do_fdt (cmd_tbl_t * cmdtp, int flag, int argc, char * const argv[])
 			}
 		}
 
+		return CMD_RET_SUCCESS;
+	}
+
+	if (!working_fdt) {
+		puts(
+			"No FDT memory address configured. Please configure\n"
+			"the FDT address via \"fdt addr <address>\" command.\n"
+			"Aborting!\n");
+		return CMD_RET_FAILURE;
+	}
+
 	/*
 	 * Move the working_fdt
 	 */
-	} else if (strncmp(argv[1], "mo", 2) == 0) {
+	if (strncmp(argv[1], "mo", 2) == 0) {
 		struct fdt_header *newaddr;
 		int  len;
 		int  err;