domains: c: fix use of unitialized variable

_fill_empty was not updating the state with the value of line, so in
recursive calls it would fail the assert checking for its own
initialization.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>

sphinx/domains/c.py

@@ -1533,6 +1533,7 @@ def _fill_empty(self, declaration: ASTDeclaration, docname: str, line: int) -> N
         self.declaration = declaration
         self.declaration.symbol = self
         self.docname = docname
+        self.line = line
         self._assert_invariants()
         # and symbol addition should be done as well
         self._add_function_params()