Fix another implicit cast of boot to int

Use proper comparison to nullptr.

It is important to use nullptr since NULL is actually an integer,
which leads to another type of warnings.

tests/gtests/blenlib/BLI_polyfill_2d_test.cc

@@ -111,7 +111,7 @@ static void test_polyfill_topology(const float poly[][2],
     const unsigned int v1 = i;
     const unsigned int v2 = (i + 1) % poly_tot;
     void **p = BLI_edgehash_lookup_p(edgehash, v1, v2);
-    EXPECT_EQ((void *)p != NULL, 1);
+    EXPECT_NE((void *)p, nullptr);
     EXPECT_EQ((intptr_t)*p, 1);
   }