Support low diffs like those on testnet

src/libckpool.c

@@ -1982,6 +1982,8 @@ double diff_from_nbits(char *nbits)
 
 	pow = nbits[0];
 	powdiff = (8 * (0x1d - 3)) - (8 * (pow - 3));
+	if (powdiff < 8)
+		powdiff = 8;
 	diff32 = be32toh(*((uint32_t *)nbits)) & 0x00FFFFFF;
 	numerator = 0xFFFFULL << powdiff;
 	return numerator / (double)diff32;