Assert CPubKey::ValidLength to the pubkey's header-relevent size

Previously this was an inline test where the specificity was probably judged
overly specific. As a class method it makes sense to maintain consistency.

And replace some magic values with their constant equivalents.

Zcash: Excludes changes to the following functions we don't have:

- ExtractPubKey (bitcoin/bitcoin#6415)
- IsCompressedPubKey (bitcoin/bitcoin#8499)

src/pubkey.h

@@ -70,6 +70,11 @@ private:
     }
 
 public:
+
+    bool static ValidSize(const std::vector<unsigned char> &vch) {
+      return vch.size() > 0 && GetLen(vch[0]) == vch.size();
+    }
+
     //! Construct an invalid public key.
     CPubKey()
     {

src/script/interpreter.cpp

@@ -65,17 +65,17 @@ static inline void popstack(vector<valtype>& stack)
 }
 
 bool static IsCompressedOrUncompressedPubKey(const valtype &vchPubKey) {
-    if (vchPubKey.size() < 33) {
+    if (vchPubKey.size() < CPubKey::COMPRESSED_PUBLIC_KEY_SIZE) {
         //  Non-canonical public key: too short
         return false;
     }
     if (vchPubKey[0] == 0x04) {
-        if (vchPubKey.size() != 65) {
+        if (vchPubKey.size() != CPubKey::PUBLIC_KEY_SIZE) {
             //  Non-canonical public key: invalid length for uncompressed key
             return false;
         }
     } else if (vchPubKey[0] == 0x02 || vchPubKey[0] == 0x03) {
-        if (vchPubKey.size() != 33) {
+        if (vchPubKey.size() != CPubKey::COMPRESSED_PUBLIC_KEY_SIZE) {
             //  Non-canonical public key: invalid length for compressed key
             return false;
         }

src/script/standard.cpp

@@ -110,7 +110,7 @@ bool Solver(const CScript& scriptPubKey, txnouttype& typeRet, std::vector<std::v
             // Template matching opcodes:
             if (opcode2 == OP_PUBKEYS)
             {
-                while (vch1.size() >= 33 && vch1.size() <= 65)
+                while (CPubKey::ValidSize(vch1))
                 {
                     vSolutionsRet.push_back(vch1);
                     if (!script1.GetOp(pc1, opcode1, vch1))
@@ -124,7 +124,7 @@ bool Solver(const CScript& scriptPubKey, txnouttype& typeRet, std::vector<std::v
 
             if (opcode2 == OP_PUBKEY)
             {
-                if (vch1.size() < 33 || vch1.size() > 65)
+                if (!CPubKey::ValidSize(vch1))
                     break;
                 vSolutionsRet.push_back(vch1);
             }