Cannot start a new section that's already on the stack

This is only relevant for FRAGMENT or UNION sections, since
normal ones would fail with "Section already defined previously".

Fixes #730

src/asm/section.c

@@ -87,15 +87,10 @@ static inline void reserveSpace(uint32_t delta_size)
 
 struct Section *out_FindSectionByName(const char *name)
 {
-	struct Section *sect = pSectionList;
-
-	while (sect) {
+	for (struct Section *sect = pSectionList; sect; sect = sect->next) {
 		if (strcmp(name, sect->name) == 0)
 			return sect;
-
-		sect = sect->next;
 	}
-
 	return NULL;
 }
 
@@ -376,6 +371,11 @@ void out_NewSection(char const *name, uint32_t type, uint32_t org,
 	if (currentLoadSection)
 		fatalerror("Cannot change the section within a `LOAD` block\n");
 
+	for (struct SectionStackEntry *stack = sectionStack; stack; stack = stack->next) {
+		if (stack->section && !strcmp(name, stack->section->name))
+			fatalerror("Section '%s' is already on the stack\n", name);
+	}
+
 	struct Section *sect = getSection(name, type, org, attribs, mod);
 
 	changeSection();

test/asm/new-pushed-section.asm

@@ -0,0 +1,7 @@
+SECTION FRAGMENT "A", ROM0
+	db 1
+PUSHS
+SECTION FRAGMENT "A", ROM0
+	db 2
+POPS
+	db 3

test/asm/new-pushed-section.err

@@ -0,0 +1,2 @@
+FATAL: new-pushed-section.asm(4):
+    Section 'A' is already on the stack