use INTERNAL_ERROR and simplify overflow check during extent merge

Author: rootvector2

src/read.c

@@ -1535,16 +1535,20 @@ static avifResult avifDecoderItemRead(avifDecoderItem * item,
         } else {
             AVIF_ASSERT_OR_RETURN(item->ownsMergedExtents);
             AVIF_ASSERT_OR_RETURN(front);
+            size_t writeOffset = (size_t)(front - item->mergedExtents.data);
             // Validate that the write will not exceed the allocated buffer
-            if ((size_t)(front - item->mergedExtents.data) > item->mergedExtents.size ||
-                bytesToRead > item->mergedExtents.size - (size_t)(front - item->mergedExtents.data)) {
-                avifDiagnosticsPrintf(diag,
-                                      "Item ID %u extent would overflow merge buffer (buffer size: %zu, current offset: %zu, bytes to write: %zu)",
-                                      item->id,
-                                      item->mergedExtents.size,
-                                      (size_t)(front - item->mergedExtents.data),
-                                      bytesToRead);
-                return AVIF_RESULT_BMFF_PARSE_FAILED;
+            if (writeOffset > item->mergedExtents.size ||
+                bytesToRead > item->mergedExtents.size - writeOffset) {
+
+                avifDiagnosticsPrintf(
+                    diag,
+                    "Item ID %u extent would overflow merge buffer (buffer size: %zu, current offset: %zu, bytes to write: %zu)",
+                    item->id,
+                    item->mergedExtents.size,
+                    writeOffset,
+                    bytesToRead);
+
+                return AVIF_RESULT_INTERNAL_ERROR;
             }
             memcpy(front, offsetBuffer.data, bytesToRead);
             front += bytesToRead;