Fix condition for assuming at EOF

Part of that condition's purpose is to ensure that we read the correct
lexer state; but it's possible now for the fstack to be non-empty
*before* the lexer state is registered, i.e. if there is an error
in the function that registers it.
This causes a NULL pointer deref.

test/asm/notexist.err

@@ -0,0 +1,4 @@
+error: at top level:
+    Failed to stat file "notexist.asm": No such file or directory
+FATAL: at top level:
+    Failed to open main file