fuzzing REFACTOR fuzz harneses (#1126)

Fix minor issues with implicit function declarations, terminate fuzzer
string inputs with string termination characters and add an XML parser
harness
diff --git a/tests/fuzz/main.c b/tests/fuzz/main.c
index 0d6c20f..2776dea 100644
--- a/tests/fuzz/main.c
+++ b/tests/fuzz/main.c
@@ -24,4 +24,18 @@
 	return 0;
 }
 
+#else
+int main(void) {
+	int ret;
+	uint8_t buf[64 * 1024];
+
+	ret = fread(buf, 1, sizeof(buf), stdin);
+	if (ret < 0) {
+		return 0;
+	}
+
+	LLVMFuzzerTestOneInput(buf, ret);
+
+	return 0;
+}
 #endif /* __AFL_COMPILER */