Changed the -sampleValue: and -samplePointer: methods to return nil/NULL instead of failing an assertion when the index parameter is out of range.
Changed the -sampleValue: and -samplePointer: methods to return nil/NULL instead of failing an assertion when the index parameter is out of range.

eoinnorris/core-plot,sureshdustrise/core-plot,divyavamsee/core-plot,eoinnorris/core-plot,sureshdustrise/core-plot,sureshdustrise/core-plot,divyavamsee/core-plot,zenXian/core-plot,tectronics/core-plot,kamalvenkat/core-plot,kamalvenkat/core-plot,tectronics/core-plot,LiDechao/core-plot,LiDechao/core-plot,zenXian/core-plot,LiDechao/core-plot,sureshdustrise/core-plot,divyavamsee/core-plot,eoinnorris/core-plot,divyavamsee/core-plot,kamalvenkat/core-plot,tectronics/core-plot,zenXian/core-plot,zenXian/core-plot,LiDechao/core-plot,kamalvenkat/core-plot,tectronics/core-plot,eoinnorris/core-plot