setopt: fix check for CURLOPT_PROXY_TLSAUTH_TYPE value

Prior to this change CURLOPT_PROXY_TLSAUTH_TYPE would return
CURLE_BAD_FUNCTION_ARGUMENT on any type other than NULL. Since there is
only one type of TLS auth and it is also the default (SRP) the TLS auth
would work anyway.

Closes #12981

lib/setopt.c

@@ -2864,13 +2864,13 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
 #endif
   case CURLOPT_TLSAUTH_TYPE:
     argptr = va_arg(param, char *);
-    if(argptr && !strncasecompare(argptr, "SRP", strlen("SRP")))
+    if(argptr && !strcasecompare(argptr, "SRP"))
       return CURLE_BAD_FUNCTION_ARGUMENT;
     break;
 #ifndef CURL_DISABLE_PROXY
   case CURLOPT_PROXY_TLSAUTH_TYPE:
     argptr = va_arg(param, char *);
-    if(argptr || !strncasecompare(argptr, "SRP", strlen("SRP")))
+    if(argptr && !strcasecompare(argptr, "SRP"))
       return CURLE_BAD_FUNCTION_ARGUMENT;
     break;
 #endif