Do not allow defining labels outside sections (#2013)

Creating a `Symbol` with `type = SYM_LABEL` but `section = nullptr`
is inconsistent and dangerous. I was not able to cause any buggy
behavior so far, but it's safer and reasonable to not create such
a symbol in the first place.

The main consequence is that `DEF(LabelOutsideSection)` will now
evaluate as 0.

src/asm/rpn.cpp

@@ -227,9 +227,9 @@ static int32_t tryConstMask(Expression const &lhs, Expression const &rhs) {
 		return -1;
 	}
 
-	assume(sym.isNumeric());
+	assume(sym.type == SYM_LABEL);
+	// We can now safely use `expr.value()` and `sym.getSection()`
 
-	// We can now safely use `expr.value()`
 	int32_t mask = expr.value();
 
 	// The mask must not cover any unknown bits

src/asm/symbol.cpp

@@ -537,6 +537,12 @@ Symbol *sym_AddVar(InternedStr symName, int32_t value) {
 static Symbol *addLabel(InternedStr symName) {
 	assumeAlreadyExpanded(symName);
 
+	Section *section = sect_GetSymbolSection();
+	if (!section) {
+		error("Cannot define label `%s` outside of a `SECTION`", symName.c_str());
+		return nullptr;
+	}
+
 	Symbol *sym = sym_FindExactSymbol(symName);
 
 	if (!sym) {
@@ -547,6 +553,7 @@ static Symbol *addLabel(InternedStr symName) {
 	} else {
 		updateSymbolFilename(*sym);
 	}
+
 	// If the symbol already exists as a ref, just "take over" it
 	sym->type = SYM_LABEL;
 	sym->data = static_cast<int32_t>(sect_GetSymbolOffset());
@@ -554,11 +561,7 @@ static Symbol *addLabel(InternedStr symName) {
 	if (options.exportAll && !symName.str().starts_with('!')) {
 		sym->isExported = true;
 	}
-	sym->section = sect_GetSymbolSection();
-
-	if (sym && !sym->section) {
-		error("Label `%s` created outside of a `SECTION`", symName.c_str());
-	}
+	sym->section = section;
 
 	return sym;
 }