Blender Git Loki

BLI_dynstr_vappendf() was crashing with strings above 256 chars, this happens with some long reports.
Problem was using va_list value more then once, fix by using va_copy().

Note, va_copy() is c99 spec but only alternative I can see is to turn BLI_dynstr_vappendf() into a macro which calls va_start/end inline.