Fix a few "Uninitialized scalar field" warnings

Fix a few warnings reported by Coverity.
None of these is critical, but making sure that class fields are
initialized can avoid heisenbugs.

src/key.h

@@ -189,7 +189,7 @@ private:
 public:
 
     // Construct an invalid private key.
-    CKey() : fValid(false) {
+    CKey() : fValid(false), fCompressed(false) {
         LockObject(vch);
     }