Add tests for function pointers.

tests/funcptr-null-4.ispc

@@ -0,0 +1,29 @@
+
+export uniform int width() { return programCount; }
+
+typedef float (*FuncType)(float, float);
+
+float foo(float a, float b) {
+    return a+b;
+}
+
+static float bar(float a, float b) {
+    return min(a, b);
+}
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = aFOO[programIndex]; 
+    float b = aFOO[0]-1;
+    FuncType func = NULL;
+    if (a == 2)
+        func = foo;
+    if (func != NULL)
+        RET[programIndex] = func(a, b);
+    else
+        RET[programIndex] = -1;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = -1;
+    RET[1] = 2;
+}