fix: possible leak on base64 decode failure

[WilliamHCarter]

decoder.decode can fail on malformed base64 input after text has already been allocated, this adds an errdefer so the temporary allocation is released if decode fails before ownership transfer.

for repro I used a simple test to confirm the leak, I didn't add this to the pr so as not to bloat the test set:

test "parse: osc52 paste with malformed base64 does not leak" {
const alloc = testing.allocator_instance.allocator();
// "!!!!" is not valid base64; decode() will return error.InvalidCharacter
const input = "\x1b]52;c;!!!!\x1b\";
var parser: Parser = .{};
try testing.expectError(error.InvalidCharacter, parser.parse(input, alloc));
}