refactor: add self-assign checks to classes which violate the clang-t…

…idy check

Both of these cases appear to be harmless, but adding the tests allows us to
turn on the aggressive clang-tidy checks.

src/arith_uint256.h

@@ -43,8 +43,10 @@ class base_uint
 
     base_uint& operator=(const base_uint& b)
     {
-        for (int i = 0; i < WIDTH; i++)
-            pn[i] = b.pn[i];
+        if (this != &b) {
+            for (int i = 0; i < WIDTH; i++)
+                pn[i] = b.pn[i];
+        }
         return *this;
     }
 

src/key.h

@@ -75,13 +75,15 @@ class CKey
 
     CKey& operator=(const CKey& other)
     {
-        if (other.keydata) {
-            MakeKeyData();
-            *keydata = *other.keydata;
-        } else {
-            ClearKeyData();
+        if (this != &other) {
+            if (other.keydata) {
+                MakeKeyData();
+                *keydata = *other.keydata;
+            } else {
+                ClearKeyData();
+            }
+            fCompressed = other.fCompressed;
         }
-        fCompressed = other.fCompressed;
         return *this;
     }