orcexecutor: Don't use negative values on arrays
orcexecutor: Don't use negative values on arrays

orc_program_find_var_by_name() can return -1 on errors, if that
happens, don't try to read/write in random memory :)

CID #1146974
CID #1146975
CID #1146976

ijsf/OpenWebRTC-orc,jpakkane/orc,ijsf/OpenWebRTC-orc,mojaves/orc,jpakkane/orc,jpakkane/orc,ijsf/OpenWebRTC-orc,mojaves/orc,mojaves/orc