Do not test incr line count on fuzzed grids (#2160)

Add an explanation about the tamed check.

BUG=oss-fuzz:68972

tests/gtest/avifincrtest_helpers.cc

@@ -87,15 +87,16 @@ uint32_t GetMinDecodedRowCount(uint32_t height, uint32_t cell_height,
   if (available_byte_count >= byte_count) {
     return height;
   }
-  // All but one cell should be decoded if at most 10 bytes are missing.
-  if ((available_byte_count + 10) >= byte_count) {
-    return height - cell_height;
-  }
 
   // The tests below can be hard to tune for any kind of input, especially
-  // fuzzed grids. Early exit in that case.
+  // fuzzed grids, where tile ordering is unknown. Early exit in that case.
   if (!enable_fine_incremental_check) return 0;
 
+  // There is no valid AV1 payload smaller than 10 bytes, so all but one cell
+  // should be decoded if at most 10 bytes are missing.
+  if ((available_byte_count + 10) >= byte_count) {
+    return height - cell_height;
+  }
   // Subtract the header because decoding it does not output any pixel.
   // Most AVIF headers are below 500 bytes.
   if (available_byte_count <= 500) {