fix(diagnostic): Don't panic on empty spans

There is another level to this bug where we better point to where the
error occurs.

src/cargo/util/toml/mod.rs

@@ -121,7 +121,7 @@ fn read_manifest_from_str(
                 .rfind('\n')
                 .map(|s| s + 1)
                 .unwrap_or(0);
-            let source_end = contents[span.end - 1..]
+            let source_end = contents[span.end.saturating_sub(1)..]
                 .find('\n')
                 .map(|s| s + span.end)
                 .unwrap_or(contents.len());

tests/testsuite/diagnostics.rs

@@ -18,6 +18,13 @@ edition = "2021"
 
     p.cargo("check")
         .with_status(101)
-        .with_stderr_contains("attempt to subtract with overflow")
+        .with_stderr(
+            "\
+error: invalid type: map, expected a sequence
+--> Cargo.toml:1:1
+ |
+ |
+",
+        )
         .run();
 }