Add missing NULL check in CanBeType

tests_ispcpp/error_0.ispc

@@ -1,6 +1,6 @@
 //@error
 //assigning mismatched polymorphic types
 
-export void foo(floating$0 bar) {
+export void foo(uniform floating$0 bar) {
     floating$1 baz = bar;
 }

tests_ispcpp/error_2.ispc

@@ -1,6 +1,6 @@
 //@error
 //assigning mismatched polymorphic types
 
-export void foo(number$0 bar) {
+export void foo(uniform floating$0 bar) {
-    floating$0 baz = bar;
+    integer baz = bar;
 }

type.cpp

@@ -855,6 +855,7 @@ PolyType::CanBeType(const Type *t) const {
 
     const AtomicType *at = CastType<AtomicType>(t);
 
+    if (at) {
         switch (restriction) {
             case TYPE_INTEGER:
                 return at->IsIntType();
@@ -865,6 +866,7 @@ PolyType::CanBeType(const Type *t) const {
             default:
                 FATAL("Unmatched case for polymorphic restriction");
         }
+    }
 
     // not an atomic type or polymorphic type
     return false;