Mark rowPointers as volatile to satisfy -Wclobbered

apps/shared/avifpng.c

diff --git a/apps/shared/avifpng.c b/apps/shared/avifpng.c
index b34e361..87dab49 100644
--- a/apps/shared/avifpng.c
+++ b/apps/shared/avifpng.c
@@ -14,7 +14,7 @@
     avifBool readResult = AVIF_FALSE;
     png_structp png = NULL;
     png_infop info = NULL;
-    png_bytep * rowPointers = NULL;
+    png_bytep * volatile rowPointers = NULL; // volatile avoids -Wclobbered due to libpng's setjmp
 
     avifRGBImage rgb;
     memset(&rgb, 0, sizeof(avifRGBImage));
@@ -139,7 +139,7 @@
     avifBool writeResult = AVIF_FALSE;
     png_structp png = NULL;
     png_infop info = NULL;
-    png_bytep * rowPointers = NULL;
+    png_bytep * volatile rowPointers = NULL; // volatile avoids -Wclobbered due to libpng's setjmp
 
     avifRGBImage rgb;
     memset(&rgb, 0, sizeof(avifRGBImage));