[flang] Fix #else with trailing text

Fixed the issue, where the extra text on #else line (' Z' in the example
below) caused the data from the "else" clause to be processed together
with the data of "then" clause.

```
      PARAMETER(A=2)
      PARAMETER(A=3)
      end
```

Author: eugeneepshteyn

flang/lib/Parser/preprocessor.cpp

@@ -684,7 +684,9 @@ void Preprocessor::Directive(const TokenSequence &dir, Prescanner &prescanner) {
             dir.GetIntervalProvenanceRange(j, tokens - j),
             "#else: excess tokens at end of directive"_port_en_US);
       }
-    } else if (ifStack_.empty()) {
+    }
+
+    if (ifStack_.empty()) {
       prescanner.Say(dir.GetTokenProvenanceRange(dirOffset),
           "#else: not nested within #if, #ifdef, or #ifndef"_err_en_US);
     } else if (ifStack_.top() != CanDeadElseAppear::Yes) {