fix for warnings from Sparse static source code checker, mostly BKE/BLI and python functions.

- use NULL rather then 0 where possible (makes code & function calls more readable IMHO).
- set static variables and functions (exposed some unused vars/funcs).
- use func(void) rather then func() for definitions.

source/blender/python/intern/bpy_rna_callback.c

@@ -24,6 +24,7 @@
 
 
 #include "bpy_rna.h"
+#include "bpy_rna_callback.h"
 #include "bpy_util.h"
 
 #include "BLI_utildefines.h"
@@ -37,7 +38,7 @@
 #define RNA_CAPSULE_ID "RNA_HANDLE"
 #define RNA_CAPSULE_ID_INVALID "RNA_HANDLE_REMOVED"
 
-void cb_region_draw(const bContext *C, ARegion *UNUSED(ar), void *customdata)
+static void cb_region_draw(const bContext *C, ARegion *UNUSED(ar), void *customdata)
 {
 	PyObject *cb_func, *cb_args, *result;
 	PyGILState_STATE gilstate;