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.

include/asm/lexer.hpp

@@ -131,6 +131,7 @@ static inline void lexer_SetGfxDigits(char const digits[4]) {
 	gfxDigits[3] = digits[3];
 }
 
+bool lexer_AtTopLevel();
 void lexer_RestartRept(uint32_t lineNo);
 void lexer_Init();
 void lexer_SetMode(LexerMode mode);

src/asm/fstack.cpp

@@ -73,7 +73,7 @@ std::string const &FileStackNode::dump(uint32_t curLineNo) const {
 }
 
 void fstk_DumpCurrent() {
-	if (contextStack.empty()) {
+	if (lexer_AtTopLevel()) {
 		fputs("at top level", stderr);
 		return;
 	}

src/asm/lexer.cpp

@@ -338,6 +338,10 @@ static bool isWhitespace(int c) {
 static LexerState *lexerState = nullptr;
 static LexerState *lexerStateEOL = nullptr;
 
+bool lexer_AtTopLevel() {
+	return lexerState == nullptr;
+}
+
 void LexerState::clear(uint32_t lineNo_) {
 	mode = LEXER_NORMAL;
 	atLineStart = true; // yylex() will init colNo due to this

test/asm/nonexist-include.asm

@@ -0,0 +1 @@
+INCLUDE "nonexist-include.inc"

test/asm/nonexist-include.err

@@ -0,0 +1,3 @@
+error: nonexist-include.asm(1):
+    Unable to open included file 'nonexist-include.inc': No such file or directory
+error: Assembly aborted (1 error)!

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

test/asm/test.sh

@@ -60,13 +60,13 @@ else
 	rm -f version.asm
 fi
 
-for i in *.asm; do
+for i in *.asm notexist.asm; do
 	flags=${i%.asm}.flags
 	RGBASMFLAGS=-Weverything
 	if [ -f "$flags" ]; then
 		RGBASMFLAGS="$(head -n 1 "$flags")" # Allow other lines to serve as comments
 	fi
-	for variant in '' '.pipe'; do
+	for variant in '' ' piped'; do
 		(( tests++ ))
 		echo "${bold}${green}${i%.asm}${variant}...${rescolors}${resbold}"
 		if [ -e "${i%.asm}.out" ]; then
@@ -85,8 +85,9 @@ for i in *.asm; do
 			desired_errput=$desired_errname
 		else
 			# `include-recursion.asm` refers to its own name inside the test code.
-			# Skip testing with stdin input for that file.
-			if [ "$i" = "include-recursion.asm" ]; then
+			# "notexist" doesn't exist, so there's no point in trying to `cat` it.
+			# Skip testing with stdin input for those file.
+			if [[ "$i" = include-recursion.asm || "$i" = notexist.asm ]]; then
 				continue
 			fi