Prevent integer overflow in ReadVarInt.

We don't normally use ReadVarInt from untrusted inputs, but we might
 see this in the case of corruption.

This is exposed in test_bitcoin_fuzzy.

src/serialize.h

@@ -336,11 +336,18 @@ I ReadVarInt(Stream& is)
     I n = 0;
     while(true) {
         unsigned char chData = ser_readdata8(is);
+        if (n > (std::numeric_limits<I>::max() >> 7)) {
+           throw std::ios_base::failure("ReadVarInt(): size too large");
+        }
         n = (n << 7) | (chData & 0x7F);
-        if (chData & 0x80)
+        if (chData & 0x80) {
+            if (n == std::numeric_limits<I>::max()) {
+                throw std::ios_base::failure("ReadVarInt(): size too large");
+            }
             n++;
-        else
+        } else {
             return n;
+        }
     }
 }