Fix RGBLINK ignoring alignment constraints when placing empty sections

Fixes #2095

src/link/assign.cpp

@@ -256,12 +256,18 @@ static void placeSection(Section &section) {
 
 	// Specially handle 0-byte SECTIONs, as they can't overlap anything
 	if (section.size == 0) {
-		// Unless the SECTION's address was fixed, the starting address
-		// is fine for any alignment, as checked in sect_DoSanityChecks.
+		// Unless the SECTION has a fixed address or non-zero alignment, the starting
+		// address is fine for any alignment, as checked in `sect_DoSanityChecks`.
 		MemoryLocation location = {
 		    .address = section.isAddressFixed ? section.org : typeInfo.startAddr,
 		    .bank = section.isBankFixed ? section.bank : typeInfo.firstBank,
 		};
+		if (section.isAlignFixed && !section.isAddressFixed) {
+			if (uint16_t offset = (location.address - section.alignOfs) & section.alignMask;
+			    offset != 0) {
+				location.address += section.alignMask + 1 - offset;
+			}
+		}
 		assignSection(section, location);
 		return;
 	}

test/link/zero-byte-sect-constraints.asm

@@ -0,0 +1,23 @@
+; Sections going into space already reserved.
+SECTION "address", ROM0[$0135]
+	assert @ == $0135
+
+SECTION "align16", ROM0, ALIGN[16,$0123] ; Should be equivalent to the above.
+	assert @ == $0123
+
+SECTION "align", ROM0, ALIGN[8,42]
+	assert @ == 42 ; Assuming that it goes into the first suitable location.
+
+
+SECTION "om nom nom", ROM0[0]
+	ds $200 ; Filling the first part of ROM0, so that the above don't land in a “free space” block but the next do.
+
+
+SECTION "free address", ROM0[$2468]
+	assert @ == $2468
+
+SECTION "free align16", ROM0, ALIGN[16,$2222] ; Should be equivalent to the above.
+	assert @ == $2222
+
+SECTION "free align", ROM0, ALIGN[13,$1234] ; Has more than one suitable location, so cannot be trivially solved.
+	assert @ == $1234 ; Assuming that it goes into the first suitable location.