make sure we catch UB in match place even with _ pattern

Author: RalfJung

tests/fail/dangling_pointers/dangling_pointer_project_underscore_match.rs

@@ -0,0 +1,15 @@
+// Make sure we find these even with many checks disabled.
+//@compile-flags: -Zmiri-disable-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-disable-validation
+
+fn main() {
+    let p = {
+        let b = Box::new(42);
+        &*b as *const i32 as *const (u8, u8, u8, u8)
+    };
+    unsafe {
+        match (*p).1 {
+            //~^ ERROR: out-of-bounds pointer arithmetic
+            _ => {}
+        }
+    }
+}

tests/fail/dangling_pointers/dangling_pointer_project_underscore_match.stderr

@@ -0,0 +1,25 @@
+error: Undefined Behavior: out-of-bounds pointer arithmetic: ALLOC has been freed, so this pointer is dangling
+  --> $DIR/dangling_pointer_project_underscore_match.rs:LL:CC
+   |
+LL |         match (*p).1 {
+   |               ^^^^^^ out-of-bounds pointer arithmetic: ALLOC has been freed, so this pointer is dangling
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+help: ALLOC was allocated here:
+  --> $DIR/dangling_pointer_project_underscore_match.rs:LL:CC
+   |
+LL |         let b = Box::new(42);
+   |                 ^^^^^^^^^^^^
+help: ALLOC was deallocated here:
+  --> $DIR/dangling_pointer_project_underscore_match.rs:LL:CC
+   |
+LL |     };
+   |     ^
+   = note: BACKTRACE (of the first span):
+   = note: inside `main` at $DIR/dangling_pointer_project_underscore_match.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+