binman: Correct handling of zero bss size

Fix the check for the __bss_size symbol, since it may be 0. Unfortunately
there was no test coverage for this.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/elf_test.py

@@ -371,6 +371,11 @@ class TestElf(unittest.TestCase):
             elf.GetSymbolOffset(fname, 'embed')
         self.assertIn('__image_copy_start', str(e.exception))
 
+    def test_get_symbol_address(self):
+        fname = self.ElfTestFile('embed_data')
+        addr = elf.GetSymbolAddress(fname, 'region_size')
+        self.assertEqual(0, addr)
+
 
 if __name__ == '__main__':
     unittest.main()