Prevent SECTION FRAGMENT combined sizes from overflowing their uint16_t size

src/link/section.cpp

@@ -146,6 +146,15 @@ static void mergeSections(Section &target, std::unique_ptr<Section> &&other) {
 
 	case SECTION_FRAGMENT:
 		checkPieceCompat(target, *other, target.size);
+		// Check that `target.size += other->size` below will not overflow
+		if (target.size + other->size > UINT16_MAX) {
+			fatalTwoAt(
+			    target,
+			    *other,
+			    "Section \"%s\" fragments combined are larger than the GB address space",
+			    target.name.c_str()
+			);
+		}
 		// Append `other` to `target`
 		other->offset = target.size;
 		target.size += other->size;

test/link/section-fragment/size-overflow/a.asm

@@ -0,0 +1,2 @@
+SECTION FRAGMENT "output", ROM0
+ds $4000

test/link/section-fragment/size-overflow/out.err

@@ -0,0 +1,5 @@
+FATAL: Section "output" fragments combined are larger than the GB address space
+    at section-fragment/size-overflow/a.asm(1)
+    and also:
+    at section-fragment/size-overflow/a.asm(1)
+Linking aborted with 1 error

test/link/test.sh

@@ -398,6 +398,14 @@ rgblinkQuiet -o "$gbtemp" "$otemp" "$gbtemp2"
 tryCmpRom "$test"/ref.out.bin
 evaluateTest
 
+test="section-fragment/size-overflow"
+startTest
+"$RGBASM" -o "$otemp" "$test"/a.asm
+continueTest
+rgblinkQuiet "$otemp" "$otemp" "$otemp" "$otemp" 2>"$outtemp"
+tryDiff "$test"/out.err "$outtemp"
+evaluateTest
+
 test="section-fragment/jr-offset"
 startTest
 "$RGBASM" -o "$otemp" "$test"/a.asm