fix redis: check geo reply member is a string before GetString

[netliomax25-code]

ParseReplyDataArray for GeoPoint in redis/src/storages/redis/parse_reply.cpp reads the first element of each per-member sub-array of a GEORADIUS/GEOSEARCH reply with GetString(), but only checks that the sub-array is non-empty rather than that the element is a string. ReplyData::GetString() resolves through std::get_if, which returns nullptr for a non-string element, so a compromised or MITM'd Redis server that returns an integer or nested array there makes the parser dereference that null pointer while move-constructing member (a UASSERT abort under the sanitizer CI, a null-pointer dereference of a std::string in release). I added an IsString check before the read that throws ParseReplyException, matching the type checks the sibling branches already perform, and covered both the rejected and accepted reply shapes with a test.