[fpgrowth] Fix O3 option

[hellcwang]

In my computer(linux 5.15 with nvidia 1070), -O3 will make program error. But when I using -O0 will generate the correct result. I found that in void *cst_to_gputree function has no return. When enable some optimization, compiler will generate error code.

Fix it by adding return ; at the end of the fpgrowth.c:1322.