Fix crash with blurhashes with reported dimension of 0x0

This commit fixes a consistent crash noticed when visiting a particular
profile.

The crash was occuring when trying to display the blurhash of a specific Event, where the metadata claimed the image dimensions were 0px x 0px.

The null dimensions caused a division by zero to occur when scaling the image down, yielding a NaN (Not a Number) size value, which crashed the app when trying to cast that CGFloat value down to an integer.

The crash was fixed by modifying the down-scaling computations to check for invalid dimensions, and return nil. The callers were then updated to fallback to a default display dimension.

Issue repro
-------

Device: iPhone 15 simulator
iOS: 17.5
Damus: dba1799
Steps:
1. Visit the profile npub1gujeqakgt7fyp6zjggxhyy7ft623qtcaay5lkc8n8gkry4cvnrzqd3f67z
2. Check accessing the profile does not crash Damus.
3. Visit the event that had invalid 0x0 dimensions on the metadata (note1qmqdualjezamcjun23l4d9xw7529m7fee6hklgtnhack2fwznxysuzuuyz)
4. Check that Damus does not crash.

Results: Steps 2 and 4 crash 100% of the time (3/3)

Testing
--------

PASS

Device: iPhone 15 simulator
iOS: 17.5
Damus: This commit
Steps: Same as repro
Results:
1. Crash no longer occurs
2. Blurhash looks ok

Closes: #2341
Changelog-Fixed: Fix crash when viewing notes with invalid image dimension metadata
Signed-off-by: Daniel D’Aquino <[email protected]>

damus/Util/Images/ImageMetadata.swift

@@ -60,7 +60,8 @@ struct ImageMetadata: Equatable {
 
 func process_blurhash(blurhash: String, size: CGSize?) async -> UIImage? {
     let res = Task.detached(priority: .low) {
-        let size = get_blurhash_size(img_size: size ?? CGSize(width: 100.0, height: 100.0))
+        let default_size = CGSize(width: 100.0, height: 100.0)
+        let size = get_blurhash_size(img_size: size ?? default_size) ?? default_size
         guard let img = UIImage.init(blurHash: blurhash, size: size) else {
             let noimg: UIImage? = nil
             return noimg
@@ -135,7 +136,8 @@ extension UIImage {
     }
 }
 
-func get_blurhash_size(img_size: CGSize) -> CGSize {
+func get_blurhash_size(img_size: CGSize) -> CGSize? {
+    guard img_size.width > 0 && img_size.height > 0 else { return nil }
     return CGSize(width: 100.0, height: (100.0/img_size.width) * img_size.height)
 }
 
@@ -145,7 +147,7 @@ func calculate_blurhash(img: UIImage) async -> String? {
     }
 
     let res = Task.detached(priority: .low) {
-        let bhs = get_blurhash_size(img_size: img.size)
+        let bhs = get_blurhash_size(img_size: img.size) ?? CGSize(width: 100.0, height: 100.0)
         let smaller = img.resized(to: bhs)
 
         guard let blurhash = smaller.blurHash(numberOfComponents: (5,5)) else {