Fix bech32::Encode() error handling

Previously, an input with invalid characters would result in out-of-bounds
reads, potentially exposing up to 224 bytes of memory following the location of
the CHARSET constant. This commit fixes the function to return an empty string,
which is what was originally documented as happening.

src/test/bech32_tests.cpp

@@ -64,4 +64,37 @@ BOOST_AUTO_TEST_CASE(bip173_testvectors_invalid)
     }
 }
 
+BOOST_AUTO_TEST_CASE(bech32_deterministic_valid)
+{
+    for (size_t i = 0; i < 255; i++) {
+        std::vector<unsigned char> input(32, i);
+        auto encoded = bech32::Encode("a", input);
+        if (i < 32) {
+            // Valid input
+            BOOST_CHECK(!encoded.empty());
+            auto ret = bech32::Decode(encoded);
+            BOOST_CHECK(ret.first == "a");
+            BOOST_CHECK(ret.second == input);
+        } else {
+            // Invalid input
+            BOOST_CHECK(encoded.empty());
+        }
+    }
+
+    for (size_t i = 0; i < 255; i++) {
+        std::vector<unsigned char> input(43, i);
+        auto encoded = bech32::Encode("a", input);
+        if (i < 32) {
+            // Valid input
+            BOOST_CHECK(!encoded.empty());
+            auto ret = bech32::Decode(encoded);
+            BOOST_CHECK(ret.first == "a");
+            BOOST_CHECK(ret.second == input);
+        } else {
+            // Invalid input
+            BOOST_CHECK(encoded.empty());
+        }
+    }
+}
+
 BOOST_AUTO_TEST_SUITE_END()