Use $ASAN_FLAGS instead of repeating its contents

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index bd95a21..abb6668 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -1193,7 +1193,7 @@
     msg "build: malloc(0) returns NULL (ASan+UBSan build)"
     scripts/config.pl full
     scripts/config.pl unset MBEDTLS_MEMORY_BUFFER_ALLOC_C
-    make CC=gcc CFLAGS="'-DMBEDTLS_CONFIG_FILE=\"$PWD/tests/configs/config-wrapper-malloc-0-null.h\"' -O -Werror -Wall -Wextra -fsanitize=address,undefined" LDFLAGS='-fsanitize=address,undefined'
+    make CC=gcc CFLAGS="'-DMBEDTLS_CONFIG_FILE=\"$PWD/tests/configs/config-wrapper-malloc-0-null.h\"' -O $ASAN_CFLAGS" LDFLAGS="$ASAN_CFLAGS"
 
     msg "test: malloc(0) returns NULL (ASan+UBSan build)"
     make test