Fix bug: Attempt to insert into immutable dictionary.

For some reason this bug only manifests itself in release mode, on Xcode 12.
Fixes Github issue 817

Bugsnag/BSGOutOfMemoryWatchdog.m

@@ -274,17 +274,19 @@ - (NSDictionary *)readSentinelFile {
         bsg_log_err(@"Failed to read oom watchdog file: %@", error);
         return nil;
     }
-    NSDictionary *contents = [BSGJSONSerialization JSONObjectWithData:data options:0 error:&error];
+    NSMutableDictionary *contents = [[BSGJSONSerialization JSONObjectWithData:data options:0 error:&error] mutableCopy];
     if (error) {
         bsg_log_err(@"Failed to read oom watchdog file: %@", error);
         return nil;
     }
 
+    NSMutableDictionary *app = [contents[BSGKeyApp] mutableCopy];
     // Override JSON data with KV store data
-    contents[BSGKeyApp][APP_KEY_IS_MONITORING_OOM] = [self.kvStore NSBooleanForKey:KV_KEY_IS_MONITORING_OOM defaultValue:false];
-    contents[BSGKeyApp][APP_KEY_IS_ACTIVE] = [self.kvStore NSBooleanForKey:KV_KEY_IS_ACTIVE defaultValue:false];
-    contents[BSGKeyApp][APP_KEY_IS_IN_FOREGROUND] = [self.kvStore NSBooleanForKey:KV_KEY_IS_IN_FOREGROUND defaultValue:false];
-    contents[BSGKeyDevice][DEVICE_KEY_LAST_LOW_MEMORY_WARNING] = [self.kvStore stringForKey:KV_KEY_LAST_LOW_MEMORY_WARNING defaultValue:@""];
+    app[APP_KEY_IS_MONITORING_OOM] = [self.kvStore NSBooleanForKey:KV_KEY_IS_MONITORING_OOM defaultValue:false];
+    app[APP_KEY_IS_ACTIVE] = [self.kvStore NSBooleanForKey:KV_KEY_IS_ACTIVE defaultValue:false];
+    app[APP_KEY_IS_IN_FOREGROUND] = [self.kvStore NSBooleanForKey:KV_KEY_IS_IN_FOREGROUND defaultValue:false];
+    app[DEVICE_KEY_LAST_LOW_MEMORY_WARNING] = [self.kvStore stringForKey:KV_KEY_LAST_LOW_MEMORY_WARNING defaultValue:@""];
+    contents[BSGKeyApp] = app;
 
     return contents;
 }