Fix wiping crash when block device is not present

Especially with /sd-ext, trying to wipe when the device is not
present causes a seg fault in the make_ext4fs function. Check to
make sure that the block device is present before trying to wipe.

Change-Id: I91973b5b832c9edbce81d61a271e84363c19e6c5
diff --git a/partition.cpp b/partition.cpp
index add9460..3fadfb9 100644
--- a/partition.cpp
+++ b/partition.cpp
@@ -1509,6 +1509,11 @@
 }
 
 bool TWPartition::Wipe_EXT4() {
+	Find_Actual_Block_Device();
+	if (!Is_Present) {
+		LOGERR("Block device not present, cannot wipe %s.\n", Display_Name.c_str());
+		return false;
+	}
 	if (!UnMount(true))
 		return false;