Rollup merge of #141239 - RalfJung:dladdr-fname, r=Noratrieb

dladdr cannot leave dli_fname to be null

There are two places in the repo calling `dladdr`, and they are inconsistent wrt their assumption of whether the `dli_fname` field can be null. Let's make them consistent. I see nothing in the docs that allows it to be null, but just to be on the safe side let's make this an assertion so hopefully we get a report if that ever happens.

compiler/rustc_session/src/filesearch.rs

@@ -82,9 +82,7 @@ fn current_dll_path() -> Result<PathBuf, String> {
                 let fname_ptr = info.dli_fname.as_ptr();
                 #[cfg(not(target_os = "cygwin"))]
                 let fname_ptr = {
-                    if info.dli_fname.is_null() {
-                        return Err("dladdr returned null pointer".into());
-                    }
+                    assert!(!info.dli_fname.is_null(), "the docs do not allow dladdr to be null");
                     info.dli_fname
                 };
                 let bytes = CStr::from_ptr(fname_ptr).to_bytes();