Only call destruction_callback if non-null (flutter#24845)

In MakeSkSurfaceFromBackingStore and other places in embedder.cc, we
call a texture or framebuffer destruction callback without first
verifying it's non-null. This adds a check before such calls.

Currently fl_renderer_gl_create_backing_store() in the Linux GTK
embedder and ExternalTextureGL::PopulateTexture() in the Windows
embedder either explicitly or implicitly set a null destruction
callback.

This prevents a crash reported when running under OpenGL ES 2.0 reported
in flutter/flutter#76881.

While this prevents the crash, it does not fix the underlying issue.

shell/platform/embedder/embedder.cc

@@ -469,7 +469,9 @@ static sk_sp<SkSurface> MakeSkSurfaceFromBackingStore(
 
   if (!surface) {
     FML_LOG(ERROR) << "Could not wrap embedder supplied render texture.";
-    texture->destruction_callback(texture->user_data);
+    if (texture->destruction_callback) {
+      texture->destruction_callback(texture->user_data);
+    }
     return nullptr;
   }
 
@@ -512,7 +514,9 @@ static sk_sp<SkSurface> MakeSkSurfaceFromBackingStore(
 
   if (!surface) {
     FML_LOG(ERROR) << "Could not wrap embedder supplied frame-buffer.";
-    framebuffer->destruction_callback(framebuffer->user_data);
+    if (framebuffer->destruction_callback) {
+      framebuffer->destruction_callback(framebuffer->user_data);
+    }
     return nullptr;
   }
   return surface;
@@ -537,7 +541,9 @@ static sk_sp<SkSurface> MakeSkSurfaceFromBackingStore(
   captures->user_data = software->user_data;
   auto release_proc = [](void* pixels, void* context) {
     auto captures = reinterpret_cast<Captures*>(context);
-    captures->destruction_callback(captures->user_data);
+    if (captures->destruction_callback) {
+      captures->destruction_callback(captures->user_data);
+    }
   };
 
   auto surface = SkSurface::MakeRasterDirectReleaseProc(
@@ -551,7 +557,9 @@ static sk_sp<SkSurface> MakeSkSurfaceFromBackingStore(
   if (!surface) {
     FML_LOG(ERROR)
         << "Could not wrap embedder supplied software render buffer.";
-    software->destruction_callback(software->user_data);
+    if (software->destruction_callback) {
+      software->destruction_callback(software->user_data);
+    }
     return nullptr;
   }
   return surface;