Auto merge of #16063 - methyl:image-usemap-panics, r=nox

Avoid panics for empty or multibyte image usemap

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Some check were added to make sure we can call `split_at` with no risk of panics (when value is empty or the first char is multibyte).

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15883 (github issue number if applicable).
- [x] There are tests for these changes

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components/script/dom/htmlimageelement.rs

@@ -423,6 +423,11 @@ impl HTMLImageElement {
         };
 
         let value = usemap_attr.value();
+
+        if value.len() == 0 || !value.is_char_boundary(1) {
+            return None
+        }
+
         let (first, last) = value.split_at(1);
 
         if first != "#" || last.len() == 0 {