glide2x,tests,display.c: silence 'diff.data' may be used uninitialized warning.

glide2x/cvg/glide/tests/display.c

@@ -222,6 +222,7 @@ int main( int argc, char **argv)
       }
     }
 
+    diff.data = NULL;
     diff.width = 0;
     diff.height = 0;
 	

glide2x/h3/glide/tests/display.c

@@ -222,6 +222,7 @@ int main( int argc, char **argv)
       }
     }
 
+    diff.data = NULL;
     diff.width = 0;
     diff.height = 0;
 	

glide2x/sst1/glide/tests/display.c

@@ -222,6 +222,7 @@ int main( int argc, char **argv)
       }
     }
 
+    diff.data = NULL;
     diff.width = 0;
     diff.height = 0;