fix build with clang6: false is not a pointer

graphics/qimageblitz/patches/patch-blitz_convolve_cpp

@@ -0,0 +1,14 @@
+$OpenBSD: patch-blitz_convolve_cpp,v 1.1 2018/04/16 15:54:14 naddy Exp $
+
+Index: blitz/convolve.cpp
+--- blitz/convolve.cpp.orig
++++ blitz/convolve.cpp
+@@ -940,7 +940,7 @@ float* BlitzPrivate::getBlurKernel(int &kernel_width, 
+     long i;
+ 
+     if(sigma == 0.0)
+-        return(false);
++        return(NULL);
+     if(kernel_width == 0)
+         kernel_width = 3;
+