Avoid copy in self-assign

I believe this was harmless, but it avoids a copy and quiets the clang-tidy
check.

Author: theuni

include/minisketch.h

@@ -239,7 +239,7 @@ class Minisketch
     /** Make this Minisketch a clone of the specified one. */
     Minisketch& operator=(const Minisketch& sketch) noexcept
     {
-        if (sketch.m_minisketch) {
+        if (this != &sketch && sketch.m_minisketch) {
             m_minisketch = std::unique_ptr<minisketch, Deleter>(minisketch_clone(sketch.m_minisketch.get()));
         }
         return *this;