Avoid a segfault on getblock if it can't read a block from disk.

src/rpcblockchain.cpp

@@ -276,7 +276,9 @@ Value getblock(const Array& params, bool fHelp)
 
     CBlock block;
     CBlockIndex* pblockindex = mapBlockIndex[hash];
-    ReadBlockFromDisk(block, pblockindex);
+
+    if(!ReadBlockFromDisk(block, pblockindex))
+        throw JSONRPCError(RPC_INTERNAL_ERROR, "Can't read block from disk");
 
     if (!fVerbose)
     {