trivial: don't check for null/false returns on dist or subprojects

contrib/ci/check-null-false-returns.py

@@ -225,6 +225,8 @@ def test_files():
     # test all C source files
     validator = ReturnValidator()
     for fn in glob.glob("**/*.c", recursive=True):
+        if fn.startswith("dist/") or fn.startswith("subprojects/"):
+            continue
         validator.parse(fn)
     for warning in validator.warnings:
         print(warning)