restored assert() in paramgrill

assert() in paramgrill are not in the benchmark path.
They should remain active, as they don't impact measurements, and their runtime is insignificant.

tests/Makefile

@@ -200,7 +200,7 @@ zstreamtest-dll : $(ZSTDDIR)/common/xxhash.c  # xxh symbols not exposed from dll
 zstreamtest-dll : $(ZSTREAM_LOCAL_FILES)
 	$(CC) $(CPPFLAGS) $(CFLAGS) $(filter %.c,$^) $(LDFLAGS) -o $@$(EXT)
 
-paramgrill : DEBUGFLAGS = -DNDEBUG  # turn off assert() for speed measurements
+paramgrill : DEBUGFLAGS =  # turn off assert() by default for speed measurements
 paramgrill : $(ZSTD_FILES) $(PRGDIR)/bench.c $(PRGDIR)/datagen.c paramgrill.c
 	$(CC) $(FLAGS) $^ -lm -o $@$(EXT)