Blender Git Loki

Fix (unreported) assert in texture code when loading some old 2.74 blend file.

Trying to open the 2.74 Fishy Cat file would generate endless asserts in
customdata code. This commit refactors and cleans up the code of
`psys_get_dupli_texture` to avoid useless calls and data access, and
validate data before trying to further access it.

No behavioral changes expected here (besides getting rid of the assert).