Add test to prevent regression

Author: inquisitivecrystal

src/test/ui/macros/issue-86082-option-env-invalid-char.rs

@@ -0,0 +1,10 @@
+// check-pass
+//
+// Regression test for issue #86082
+//
+// Checks that option_env! does not panic on receiving an invalid
+// environment variable name.
+
+fn main() {
+    option_env!("\0=");
+}