powerpc: handle error case in cpm_muram_alloc()

rh_alloc() returns (unsigned long)-ERRxx on error, which may
result in overwriting memory outside the MURAM AREA.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <scottwood@freescale.com>

arch/powerpc/sysdev/cpm_common.c

@@ -147,7 +147,8 @@ unsigned long cpm_muram_alloc(unsigned long size, unsigned long align)
 	spin_lock_irqsave(&cpm_muram_lock, flags);
 	cpm_muram_info.alignment = align;
 	start = rh_alloc(&cpm_muram_info, size, "commproc");
-	memset_io(cpm_muram_addr(start), 0, size);
+	if (!IS_ERR_VALUE(start))
+		memset_io(cpm_muram_addr(start), 0, size);
 	spin_unlock_irqrestore(&cpm_muram_lock, flags);
 
 	return start;