secret service: fix error creating credential (#996)

Multiple Linux users have reported that they are unable to use Secret
Service as their credential store, as GCM throws the following error:

sec_free: Assertion `cell->requested > 0' failed.

The root cause is that we're using the libsecret secret_value_get
function to obtain secret data, then attempting to free the string with
secret_password_free. It appears that secret_password_free is only meant
to be used to free nonpageable memory [1], however. Removing this call
fixes the issue, as verified with a successful git-credential-manager
diagnose (which was previously failing with the above error).

[1] secret_password_free manpage

https://www.manpagez.com/html/libsecret-1/libsecret-1-0.18.6/libsecret-Password-storage.php#secret-password-free

Fixes #793

Author: ldennington

src/shared/Core/Interop/Linux/SecretServiceCollection.cs

@@ -291,7 +291,6 @@ private static unsafe ICredential CreateCredentialFromItem(SecretItem* item)
                 if (accountKeyPtr != IntPtr.Zero) Marshal.FreeHGlobal(accountKeyPtr);
                 if (serviceKeyPtr != IntPtr.Zero) Marshal.FreeHGlobal(serviceKeyPtr);
                 if (value != null) secret_value_unref(value);
-                if (passwordPtr != IntPtr.Zero) secret_password_free(passwordPtr);
                 if (error != null) g_error_free(error);
             }
         }