Initialize variables which gcc6 for musl libc complains
may be unitialized when returned.

--- src/touch.cpp	2013-02-08 20:05:24.000000000 +0100
+++ src/touch.cpp	2016-10-04 10:57:05.683000000 +0200
@@ -195,7 +195,7 @@
 }
 
 float frame_touch_get_device_x(UFTouch touch) {
-  float x;
+  float x = 0.0f;
   const oif::frame::UFTouch* uftouch =
       static_cast<const oif::frame::UFTouch*>(touch);
   UFStatus status = uftouch->GetValue(UFAxisTypeX, &x);
@@ -204,7 +204,7 @@
 }
 
 float frame_touch_get_device_y(UFTouch touch) {
-  float y;
+  float y = 0.0f;
   const oif::frame::UFTouch* uftouch =
       static_cast<const oif::frame::UFTouch*>(touch);
   UFStatus status = uftouch->GetValue(UFAxisTypeY, &y);