Fix underflow of ALIGN[16, offset] (#2090)

src/asm/section.cpp

@@ -717,8 +717,17 @@ void sect_AlignPC(uint8_t alignment, uint16_t offset) {
 	} else if (alignment == 16) {
 		// Treat an alignment large enough as fixing the address.
 		// Note that this also ensures that a section's alignment never becomes 16 or greater.
-		sect->align = 0; // Reset the alignment, since we're fixing the address.
-		sect->org = offset - curOffset;
+		if (offset < curOffset) {
+			error(
+			    "Section already contains %" PRIu32
+			    " bytes, higher than this aligned address $%04" PRIx32,
+			    curOffset,
+			    offset
+			);
+		} else {
+			sect->align = 0; // Reset the alignment, since we're fixing the address.
+			sect->org = offset - curOffset;
+		}
 	} else if (alignment > sect->align) {
 		sect->align = alignment;
 		// We need `(sect->alignOfs + curOffset) & alignMask == offset`

test/asm/align-beyond-size.asm

@@ -0,0 +1,3 @@
+SECTION "test", ROM0
+db 1, 2, 3
+align 16, 2

test/asm/align-beyond-size.err

@@ -0,0 +1,3 @@
+error: Section already contains 3 bytes, higher than this aligned address $0002
+    at align-beyond-size.asm(3)
+Assembly aborted with 1 error