vfb: use calloc() instead of malloc()

Using calloc() instead of malloc() as preventive measure, so there
never can be any hidden bugs or leaks due uninitialized memory.

The extra cost of using this compiler intrinsic should be practically
impossible to measure - in many cases a good compiler can even deduce
if certain areas really don't need to be zero'd (because they're written
to right after allocation) and create more efficient machine code.

The code pathes in question are pretty cold anyways, so it's probably
not worth even thinking about potential extra runtime costs.

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

hw/vfb/InitOutput.c

@@ -692,7 +692,7 @@ vfbAllocateFramebufferMemory(vfbScreenInfoPtr pvfb)
 #endif
 
     case NORMAL_MEMORY_FB:
-        pvfb->pXWDHeader = (XWDFileHeader *) malloc(pvfb->sizeInBytes);
+        pvfb->pXWDHeader = (XWDFileHeader *) calloc(1, pvfb->sizeInBytes);
         break;
     }