Revert "Preallocate rowPointers in PNG code to avoid -Wclobbered errors"

This reverts commit 79e4c6c957aa90d7b7d8e46239ce9af40136b4c8.

apps/shared/avifpng.c

diff --git a/apps/shared/avifpng.c b/apps/shared/avifpng.c
index 5ce5177..b34e361 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 = (png_bytep *)malloc(sizeof(png_bytep) * avif->height);
+    png_bytep * rowPointers = NULL;
 
     avifRGBImage rgb;
     memset(&rgb, 0, sizeof(avifRGBImage));
@@ -112,6 +112,7 @@
     avifRGBImageSetDefaults(&rgb, avif);
     rgb.depth = imgBitDepth;
     avifRGBImageAllocatePixels(&rgb);
+    rowPointers = (png_bytep *)malloc(sizeof(png_bytep) * rgb.height);
     for (uint32_t y = 0; y < rgb.height; ++y) {
         rowPointers[y] = &rgb.pixels[y * rgb.rowBytes];
     }
@@ -126,7 +127,9 @@
     if (png) {
         png_destroy_read_struct(&png, &info, NULL);
     }
-    free(rowPointers);
+    if (rowPointers) {
+        free(rowPointers);
+    }
     avifRGBImageFreePixels(&rgb);
     return readResult;
 }
@@ -136,7 +139,7 @@
     avifBool writeResult = AVIF_FALSE;
     png_structp png = NULL;
     png_infop info = NULL;
-    png_bytep * rowPointers = (png_bytep *)malloc(sizeof(png_bytep) * avif->height);
+    png_bytep * rowPointers = NULL;
 
     avifRGBImage rgb;
     memset(&rgb, 0, sizeof(avifRGBImage));
@@ -189,6 +192,7 @@
     rgb.depth = rgbDepth;
     avifRGBImageAllocatePixels(&rgb);
     avifImageYUVToRGB(avif, &rgb);
+    rowPointers = (png_bytep *)malloc(sizeof(png_bytep) * rgb.height);
     for (uint32_t y = 0; y < rgb.height; ++y) {
         rowPointers[y] = &rgb.pixels[y * rgb.rowBytes];
     }
@@ -204,7 +208,9 @@
     if (png) {
         png_destroy_write_struct(&png, &info);
     }
-    free(rowPointers);
+    if (rowPointers) {
+        free(rowPointers);
+    }
     avifRGBImageFreePixels(&rgb);
     return writeResult;
 }