all.sh: Skip build_mingw correctly

If i686-w64-mingw32-gcc is not installed, then
build_mingw should be unsupported.

Signed-off-by: Pengyu Lv <pengyu.lv@arm.com>
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 7d91fa2..58fa4bf 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -3647,8 +3647,8 @@
     make WINDOWS_BUILD=1 clean
 }
 support_build_mingw() {
-    case $(i686-w64-mingw32-gcc -dumpversion) in
-        [0-5]*) false;;
+    case $(i686-w64-mingw32-gcc -dumpversion 2>/dev/null) in
+        [0-5]*|"") false;;
         *) true;;
     esac
 }