tools: fix TestFdtUtil()

Variable old_outdir cannot be used before assignment.
The assignment must occur before the try block.

tools/dtoc/test_fdt.py:796:26:
E0601: Using variable 'old_outdir' before assignment
(used-before-assignment)

Add missing space in assignment.

Fixes: a004f29464 ("binman: Tidy up _SetupDtb() to use its own temporary file")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

tools/dtoc/test_fdt.py

@@ -784,8 +784,8 @@ class TestFdtUtil(unittest.TestCase):
 
     def test_ensure_compiled_tmpdir(self):
         """Test providing a temporary directory"""
-        try:
         old_outdir = tools.outdir
+        try:
             tools.outdir= None
             tmpdir = tempfile.mkdtemp(prefix='test_fdt.')
             dtb = fdt_util.EnsureCompiled(find_dtb_file('dtoc_test_simple.dts'),