Properly handle sscanf return value in ./src/secp256k1/src/tests.c

A bug exists which allows the sh variable to be used uninitialized when sscanf returns EOF.

Observe:

bitcoin/src/secp256k1/src/tests.c

Lines 5019 to 5040 in 536590f

	int main(int argc, char **argv) {
	unsigned char seed16[16] = {0};
	unsigned char run32[32] = {0};
	/* find iteration count */
	if (argc > 1) {
	count = strtol(argv[1], NULL, 0);
	}
	/* find random seed */
	if (argc > 2) {
	int pos = 0;
	const char* ch = argv[2];
	while (pos < 16 && ch[0] != 0 && ch[1] != 0) {
	unsigned short sh;
	if (sscanf(ch, "%2hx", &sh)) {
	seed16[pos] = sh;
	} else {
	break;
	}
	ch += 2;
	pos++;
	}

A simple fix would be to explicitly test if sscanf's return value is equal to 1.

[fanquake]

Please send any issues in relation to secp256k1 to the upstream repository: https://github.com/bitcoin-core/secp256k1.