ndn_sig_verifier_verify_data is not reentrant

[yoursunny]

The function writes into a global variable m_userdata. Consequently, if the application invokes ndn_sig_verifier_verify_data with a new Data packet before the previous validation has completed, undefined behavior may occur.

ndn-lite/app-support/ndn-sig-verifier.c

Lines 30 to 32 in 6d433ac

	static ndn_sig_verifier_userdata_t m_userdata;
	static ndn_sig_verifier_state_t m_sig_verifier_state;
	static uint8_t verifier_buf[4096];

ndn-lite/app-support/ndn-sig-verifier.c

Lines 330 to 333 in 6d433ac

	m_userdata.is_interest = false;
	m_userdata.original_pkt = (void*)&data;
	m_userdata.on_success_cbk = on_success;
	m_userdata.on_failure_cbk = on_failure;

To solve this issue:

1. Introduce a ndn_sig_verifier_verify_data_r function that accepts a context argument to carry per-validation state, to be used in place of the global variable(s). The calling application is responsible for allocating memory (either statically or dynamically) for the context struct.
2. Implement ndn_sig_verifier_verify_data as a wrapper of ndn_sig_verifier_verify_data that uses global variable or function-scope static variable as context.
3. Update the documentation of ndn_sig_verifier_verify_data to note the non-reentrancy limitation.