AK: Don't crash on invalid Base64 input

In the long-term, we should probably have a way to signal decoding
failure. For now, it should suffice to at least not crash. This is
particularly relevant because apparently this can be triggered while
parsing a PEM certificate, which happens during every TLS connection.

Found by OSS Fuzz
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=38979

AK/Base64.cpp

@@ -61,7 +61,7 @@ ByteBuffer decode_base64(const StringView& input)
                 *is_padding = true;
             return 0;
         }
-        return table[input[offset]];
+        return table[static_cast<unsigned char>(input[offset])];
     };
 
     Vector<u8> output;