helix-term:fix crash on unwrap graphemes

This change replaces a direct unwrap with a map. If the optional value
is missing, a default is used.

The issue was discovered accidentally while porting the Helix editor to
an ARM-based platform and running it in QEMU.

helix-term/src/ui/prompt.rs

@@ -779,8 +779,8 @@ impl Component for Prompt {
             col += self.line[self.cursor..]
                 .graphemes(true)
                 .next()
-                .unwrap()
-                .width();
+                .map(|g| g.width())
+                .unwrap_or(0);
         }
 
         let line = area.height as usize - 1;