session: Handle invalid strings in keyboard prompt

If we are given no prompts, there is no reason to continue. If we are
given no instruction or username string, I've opted to use an empty
string instead, though this can be argued to be incorrect.

Fixes #326

src/session.rs

@@ -386,16 +386,29 @@ impl Session {
             // There's not much to be done with them though because the
             // signature of the callback doesn't allow reporting an error.
             let _ = catch_unwind(AssertUnwindSafe(|| {
+                if prompts.is_null() || responses.is_null() || num_prompts < 0 {
+                    return;
+                }
+
                 let prompter = unsafe { &mut **(abstrakt as *mut *mut P) };
 
-                let username =
-                    unsafe { slice::from_raw_parts(username as *const u8, username_len as usize) };
-                let username = String::from_utf8_lossy(username);
+                let username = if !username.is_null() && username_len >= 0 {
+                    let username = unsafe {
+                        slice::from_raw_parts(username as *const u8, username_len as usize)
+                    };
+                    String::from_utf8_lossy(username)
+                } else {
+                    Cow::Borrowed("")
+                };
 
-                let instruction = unsafe {
-                    slice::from_raw_parts(instruction as *const u8, instruction_len as usize)
+                let instruction = if !instruction.is_null() && instruction_len >= 0 {
+                    let instruction = unsafe {
+                        slice::from_raw_parts(instruction as *const u8, instruction_len as usize)
+                    };
+                    String::from_utf8_lossy(instruction)
+                } else {
+                    Cow::Borrowed("")
                 };
-                let instruction = String::from_utf8_lossy(instruction);
 
                 let prompts = unsafe { slice::from_raw_parts(prompts, num_prompts as usize) };
                 let responses =