CSharp: fix LDKStr array decoding

CSharp: fix LDKStr array decoding

Strings are encoded as array of string references, those references
are actually pointers to the string location and have 64bit in size,
but decoding logic was decoding those pointers as 32bit which caused
the following exception, this commit fixes that.

Exception:

Fatal error. System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
   at InternalUtils.getArrayLength(Int64)
   at InternalUtils.decodeUint8Array(Int64)
   at InternalUtils.decodeString(Int64)
   at org.ldk.structs.Result_CVec_StrZIOErrorZ+Result_CVec_StrZIOErrorZ_OK..ctor(System.Object, Int64)
   at org.ldk.structs.Result_CVec_StrZIOErrorZ.constr_from_ptr(Int64)
   at org.ldk.structs.KVStore.list(System.String, System.String)