composite: silence some warnings on possible NULL dereference

Unlikely to practically happen, but still safer to just check for sure.
A simple zero-value test doesn't cost us much (on modern CPUs perhaps
not even a full clock cycle).

| ../composite/compalloc.c: In function ‘compRedirectWindow’:
| ../composite/compalloc.c:167:35: warning: dereference of NULL ‘pClient’ [CWE-476] [-Wanalyzer-null-dereference]
|   167 |     ccw->id = FakeClientID(pClient->index);
|       |                            ~~~~~~~^~~~~~~

| ../composite/compalloc.c: In function ‘compUnredirectWindow’:
| ../composite/compalloc.c:331:75: warning: dereference of NULL ‘pClient’ [CWE-476] [-Wanalyzer-null-dereference]
|   331 |         if (ccw->update == update && dixClientIdForXID(ccw->id) == pClient->index) {
|       |                                                                    ~~~~~~~^~~~~~~

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

composite/compalloc.c

@@ -43,6 +43,8 @@
 
 #include <dix-config.h>
 
+#include "os/bug_priv.h"
+
 #include "compint.h"
 
 static Bool
@@ -133,6 +135,8 @@ compHandleMarkedWindows(WindowPtr pWin, WindowPtr pLayerWin)
 int
 compRedirectWindow(ClientPtr pClient, WindowPtr pWin, int update)
 {
+    BUG_RETURN_VAL(!pClient, BadMatch);
+
     CompWindowPtr cw = GetCompWindow(pWin);
     CompClientWindowPtr ccw;
     CompScreenPtr cs = GetCompScreen(pWin->drawable.pScreen);
@@ -323,6 +327,8 @@ compUnredirectWindow(ClientPtr pClient, WindowPtr pWin, int update)
     CompWindowPtr cw = GetCompWindow(pWin);
     CompClientWindowPtr ccw;
 
+    BUG_RETURN_VAL(!pClient, BadValue);
+
     if (!cw)
         return BadValue;