Fix malloc_stats_print(..., "a") output.

Fix the logic in stats_print() such that if the "a" flag is passed in
without the "m" flag, merged statistics will be printed even if only one
arena is initialized.

src/stats.c

@@ -748,7 +748,7 @@ stats_print(void (*write_cb)(void *, const char *), void *cbopaque,
 						ninitialized++;
 				}
 
-				if (ninitialized > 1) {
+				if (ninitialized > 1 || unmerged == false) {
 					/* Print merged arena stats. */
 					malloc_cprintf(write_cb, cbopaque,
 					    "\nMerged arenas stats:\n");