Allow gainmap_item_id to be 0 in avifinfo_fuzz

Change-Id: I840081bd14be4fdcec977063c70d837608bff7ec

tests/avifinfo_fuzz.cc

diff --git a/tests/avifinfo_fuzz.cc b/tests/avifinfo_fuzz.cc
index 9ad9b72..f561552 100644
--- a/tests/avifinfo_fuzz.cc
+++ b/tests/avifinfo_fuzz.cc
@@ -97,7 +97,7 @@
       }
       if (features.width == 0u || features.height == 0u ||
           features.bit_depth == 0u || features.num_channels == 0u ||
-          !features.has_gainmap != !features.gainmap_item_id ||
+          (!features.has_gainmap && features.gainmap_item_id) ||
           !features.primary_item_id_location !=
               !features.primary_item_id_bytes) {
         std::abort();

tests/avifinfo_fuzz_fast.cc

diff --git a/tests/avifinfo_fuzz_fast.cc b/tests/avifinfo_fuzz_fast.cc
index ee1bd46..1768472 100644
--- a/tests/avifinfo_fuzz_fast.cc
+++ b/tests/avifinfo_fuzz_fast.cc
@@ -32,7 +32,7 @@
     }
     if (features.width == 0u || features.height == 0u ||
         features.bit_depth == 0u || features.num_channels == 0u ||
-        !features.has_gainmap != !features.gainmap_item_id ||
+        (!features.has_gainmap && features.gainmap_item_id) ||
         !features.primary_item_id_location != !features.primary_item_id_bytes) {
       std::abort();
     }