descriptor: check whitespace in ParsePubkeyInner

Due to Base58, keys with whitespace at the beginning or
at the end are successfully parsed. This commit adds a
check into `ParsePubkeyInner` to verify whether if the
first or last char of the key is a space.

src/script/descriptor.cpp

@@ -1509,6 +1509,10 @@ std::vector<std::unique_ptr<PubkeyProvider>> ParsePubkeyInner(uint32_t key_exp_i
         error = "No key provided";
         return {};
     }
+    if (IsSpace(str.front()) || IsSpace(str.back())) {
+        error = strprintf("Key '%s' is invalid due to whitespace", str);
+        return {};
+    }
     if (split.size() == 1) {
         if (IsHex(str)) {
             std::vector<unsigned char> data = ParseHex(str);