Adjust defaulting logic for handling nil case of Credentials. Completes fix for #100.

Initial fix for Credentials BAD_ACCESS error.

SwiftGit2/Credentials.swift

@@ -22,8 +22,23 @@ public enum Credentials {
 	case plaintext(username: String, password: String)
 	case sshMemory(username: String, publicKey: String, privateKey: String, passphrase: String)
 
-	internal static func fromPointer(_ pointer: UnsafeMutableRawPointer) -> Credentials {
-		return Unmanaged<Wrapper<Credentials>>.fromOpaque(UnsafeRawPointer(pointer)).takeRetainedValue().value
+	private static var previouslyUsedPointer: String? = nil
+	internal static func fromPointer(_ pointer: UnsafeMutableRawPointer) -> Credentials? {
+		// check if we had just seen this pointer
+		if pointer.debugDescription == previouslyUsedPointer {
+			// we have already used this pointer, so it is likely that libgit2
+			// has already freed the memory and using Unmanaged<>.fromOpaque will
+			// result in a BAD_ACCESS crash
+			return nil
+		} else {
+			// mark that we have used this pointer so that
+			// later attempts to use it in this function will
+			// be blocked
+			previouslyUsedPointer = pointer.debugDescription
+			// access the pointer and convert it into a
+			// Credentials Swift object
+			return Unmanaged<Wrapper<Credentials>>.fromOpaque(UnsafeRawPointer(pointer)).takeRetainedValue().value
+		}
 	}
 
 	internal func toPointer() -> UnsafeMutableRawPointer {
@@ -55,6 +70,8 @@ internal func credentialsCallback(
 		result = git_cred_userpass_plaintext_new(cred, username, password)
 	case .sshMemory(let username, let publicKey, let privateKey, let passphrase):
 		result = git_cred_ssh_key_memory_new(cred, username, publicKey, privateKey, passphrase)
+	default:
+		result = git_cred_default_new(cred)
 	}
 
 	return (result != GIT_OK.rawValue) ? -1 : 0