Adds additional zero checks for setCompact

These encoded values should all decode to zero

   Positive Numbers
    0x01003456 (1 byte  0x00)
    0x02000056 (2 bytes 0x0000)
    0x03000000 (3 bytes 0x000000)
    0x04000000 (4 bytes 0x000000_00)

   Negative Numbers
    0x00923456 -(0 bytes, so zero)
    0x01803456 -(1 byte  0x00)
    0x02800056 -(2 bytes 0x0000)
    0x03800000 -(3 bytes 0x000000)
    0x04800000 -(4 bytes 0x000000_00)

src/test/bignum_tests.cpp

@@ -135,6 +135,42 @@ BOOST_AUTO_TEST_CASE(bignum_SetCompact)
     BOOST_CHECK_EQUAL(num.GetHex(), "0");
     BOOST_CHECK_EQUAL(num.GetCompact(), 0U);
 
+    num.SetCompact(0x01003456);
+    BOOST_CHECK_EQUAL(num.GetHex(), "0");
+    BOOST_CHECK_EQUAL(num.GetCompact(), 0U);
+
+    num.SetCompact(0x02000056);
+    BOOST_CHECK_EQUAL(num.GetHex(), "0");
+    BOOST_CHECK_EQUAL(num.GetCompact(), 0U);
+
+    num.SetCompact(0x03000000);
+    BOOST_CHECK_EQUAL(num.GetHex(), "0");
+    BOOST_CHECK_EQUAL(num.GetCompact(), 0U);
+
+    num.SetCompact(0x04000000);
+    BOOST_CHECK_EQUAL(num.GetHex(), "0");
+    BOOST_CHECK_EQUAL(num.GetCompact(), 0U);
+
+    num.SetCompact(0x00923456);
+    BOOST_CHECK_EQUAL(num.GetHex(), "0");
+    BOOST_CHECK_EQUAL(num.GetCompact(), 0U);
+
+    num.SetCompact(0x01803456);
+    BOOST_CHECK_EQUAL(num.GetHex(), "0");
+    BOOST_CHECK_EQUAL(num.GetCompact(), 0U);
+
+    num.SetCompact(0x02800056);
+    BOOST_CHECK_EQUAL(num.GetHex(), "0");
+    BOOST_CHECK_EQUAL(num.GetCompact(), 0U);
+
+    num.SetCompact(0x03800000);
+    BOOST_CHECK_EQUAL(num.GetHex(), "0");
+    BOOST_CHECK_EQUAL(num.GetCompact(), 0U);
+    
+    num.SetCompact(0x04800000);
+    BOOST_CHECK_EQUAL(num.GetHex(), "0");
+    BOOST_CHECK_EQUAL(num.GetCompact(), 0U);
+
     num.SetCompact(0x01123456);
     BOOST_CHECK_EQUAL(num.GetHex(), "12");
     BOOST_CHECK_EQUAL(num.GetCompact(), 0x01120000U);