Fix "prof.reset" mallctl-related corruption.

Fix heap profiling to distinguish among otherwise identical sample sites
with interposed resets (triggered via the "prof.reset" mallctl).  This
bug could cause data structure corruption that would most likely result
in a segfault.

include/jemalloc/internal/prof.h

@@ -90,10 +90,11 @@ struct prof_tctx_s {
 	prof_tdata_t		*tdata;
 
 	/*
-	 * Copy of tdata->thr_uid, necessary because tdata may be defunct during
-	 * teardown.
+	 * Copy of tdata->thr_{uid,discrim}, necessary because tdata may be
+	 * defunct during teardown.
 	 */
 	uint64_t		thr_uid;
+	uint64_t		thr_discrim;
 
 	/* Profiling counters, protected by tdata->lock. */
 	prof_cnt_t		cnts;