randr: Catch two more potential unset rrScrPriv uses

Ricardo Salveti <ricardo.salveti@linaro.org> found one place where the
randr code could use the randr screen private data without checking
for null first. This happens when the X server is running with
multiple screens, some of which are randr enabled and some of which
are not. Applications making protocol requests to the non-randr
screens can cause segfaults where the server touches the unset private
structure.

I audited the code and found two more possible problem spots; the
trick to auditing for this issue was to look for functions not taking
a RandR data structure and where there was no null screen private
check above them in the call graph.

Signed-off-by: Keith Packard <keithp@keithp.com>

randr/rroutput.c

@@ -546,7 +546,8 @@ ProcRRSetOutputPrimary(ClientPtr client)
     }
 
     pScrPriv = rrGetScrPriv(pWin->drawable.pScreen);
-    RRSetPrimaryOutput(pWin->drawable.pScreen, pScrPriv, output);
+    if (pScrPriv)
+        RRSetPrimaryOutput(pWin->drawable.pScreen, pScrPriv, output);
 
     return Success;
 }

randr/rrscreen.c

@@ -248,6 +248,9 @@ ProcRRSetScreenSize(ClientPtr client)
 
     pScreen = pWin->drawable.pScreen;
     pScrPriv = rrGetScrPriv(pScreen);
+    if (!pScrPriv)
+        return BadMatch;
+
     if (stuff->width < pScrPriv->minWidth || pScrPriv->maxWidth < stuff->width) {
         client->errorValue = stuff->width;
         return BadValue;