Fixed crash reported by Naudia Nadezda, crash when closing a texture multipreview.

Also updated LLPreviewTexture to be closer to upstream and better for translation,
added lltrans for FileSaved and Receiving,
and floater_preview_texture.xml now uses No Alpha, Has Alpha, Unknown as strings.

indra/newview/skins/default/xui/en-us/floater_preview_texture.xml

@@ -43,6 +43,8 @@
 		h_pad="50" halign="left" height="16" right="345" mouse_opaque="true" name="alphanote" v_pad="0" width="80">
 			Loading...
 	</text>
+	<string name="No Alpha">No Alpha</string>
+	<string name="Has Alpha">Has Alpha</string>
 	<button bottom="8" enabled="true" follows="left|bottom" font="SansSerif" halign="center" height="20" 
 		label="Keep" label_selected="Keep" left="9" mouse_opaque="true" name="Keep" scale_image="true" width="100" />
 	<button bottom="8" enabled="true" follows="left|bottom" font="SansSerif" halign="center" height="20" 
@@ -86,4 +88,5 @@
 			2:1
 		</combo_item>
 	</combo_box>
-</floater>
+	<string name="Unknown">Unknown</string>
+</floater>

indra/newview/skins/default/xui/en-us/strings.xml

@@ -2916,6 +2916,10 @@ Expected .wav, .tga, .bmp, .jpg, .jpeg, or .bvh
 	<string name="accel-win-alt">Alt-</string>
 	<string name="accel-win-shift">Shift-</string>
 
+	<!-- Previews -->
+	<string name="FileSaved">File Saved</string>
+	<string name="Receiving">Receiving</string>
+
 	<!-- Directions, HUD -->
 	<string name="Direction_Forward">Forward</string>
 	<string name="Direction_Left">Left</string>