examples: converting from Infallible is unreachable

Use the same implementation for both examples/error.rs and
examples/pthread_mutex.rs.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: bonzini

examples/error.rs

@@ -7,8 +7,8 @@ use std::alloc::AllocError;
 pub struct Error;
 
 impl From<Infallible> for Error {
-    fn from(_: Infallible) -> Self {
-        Self
+    fn from(e: Infallible) -> Self {
+        match e {}
     }
 }