Don't panic across FFI

Panicking across FFI was UB in older Rust versions and thus because of
MSRV it's safer to avoid it. This replaces the panic with print+abort on
`std` and double panic on no-std.

Closes rust-bitcoin#354

Author: Kixunil

secp256k1-sys/src/lib.rs

@@ -569,6 +569,33 @@ pub unsafe fn secp256k1_context_destroy(ctx: *mut Context) {
     rustsecp256k1_v0_4_1_context_destroy(ctx)
 }
 
+/// FFI-safe replacement for panic
+///
+/// Prints to stderr and aborts with `std`, double panics without `std`.
+#[cfg_attr(not(feature = "std"), allow(unused))]
+fn ffi_abort(msg: impl core::fmt::Display) -> ! {
+    #[cfg(feature = "std")]
+    {
+        eprintln!("[libsecp256k1] {}", msg);
+        std::process::abort()
+    }
+    #[cfg(not(feature = "std"))]
+    {
+        use core::fmt::Display;
+
+        // Abort by double panic
+        struct PanicOnDrop<M: Display>(M);
+
+        impl<T: Display> Drop for PanicOnDrop<T> {
+            fn drop(&mut self) {
+                panic!("[libsecp256k1] {}", self.0);
+            }
+        }
+
+        let _bomb = PanicOnDrop(&msg);
+        panic!("[libsecp256k1] {}", &msg)
+    }
+}
 
 /// **This function is an override for the C function, this is the an edited version of the original description:**
 ///
@@ -594,7 +621,7 @@ pub unsafe extern "C" fn rustsecp256k1_v0_4_1_default_illegal_callback_fn(messag
     use core::str;
     let msg_slice = slice::from_raw_parts(message as *const u8, strlen(message));
     let msg = str::from_utf8_unchecked(msg_slice);
-    panic!("[libsecp256k1] illegal argument. {}", msg);
+    ffi_abort(format_args!("illegal argument. {}", msg));
 }
 
 /// **This function is an override for the C function, this is the an edited version of the original description:**
@@ -617,7 +644,7 @@ pub unsafe extern "C" fn rustsecp256k1_v0_4_1_default_error_callback_fn(message:
     use core::str;
     let msg_slice = slice::from_raw_parts(message as *const u8, strlen(message));
     let msg = str::from_utf8_unchecked(msg_slice);
-    panic!("[libsecp256k1] internal consistency check failed {}", msg);
+    ffi_abort(format_args!("internal consistency check failed {}", msg));
 }
 
 #[cfg(not(rust_secp_no_symbol_renaming))]
@@ -826,7 +853,7 @@ mod fuzz_dummy {
             *output = 4;
             ptr::copy((*pk).0.as_ptr(), output.offset(1), 64);
         } else {
-            panic!("Bad flags");
+            ffi_abort(format_args!("Bad flags"));
         }
         1
      }