Replace deprecated Data.withUnsafeMutableBytes call

Closes: #214

damus/Nostr/NostrEvent.swift

@@ -448,7 +448,7 @@ func hex_encode(_ data: Data) -> String {
 func random_bytes(count: Int) -> Data {
     var data = Data(count: count)
     _ = data.withUnsafeMutableBytes { mutableBytes in
-        SecRandomCopyBytes(kSecRandomDefault, count, mutableBytes)
+        SecRandomCopyBytes(kSecRandomDefault, count, mutableBytes.baseAddress!)
     }
     return data
 }

damusTests/damusTests.swift

@@ -33,6 +33,13 @@ class damusTests: XCTestCase {
         }
     }
     
+    func testRandomBytes() {
+        let bytes = random_bytes(count: 32)
+        
+        print("testRandomBytes \(hex_encode(bytes))")
+        XCTAssertEqual(bytes.count, 32)
+    }
+    
     func testParseMentionWithMarkdown() {
         let md = """
         Testing markdown in damus