Removed necessity to count twice and allocate similar string

Author: pl752

src/FirebirdSql.Data.FirebirdClient/Client/Managed/Version10/GdsStatement.cs

@@ -1786,13 +1786,9 @@ private static string TruncateStringByRuneCount(string s, DbField field)
 			return s;
 		}
 
-		var runeCount = s.CountRunes();
-		if (runeCount <= field.CharCount)
-		{
-			return s;
-		}
+		var truncated = s.TruncateStringToRuneCount(field.CharCount);
 
-		return new string(s.TruncateStringToRuneCount(field.CharCount));
+		return truncated == s.AsSpan() ? s : new string(truncated);
 	}
 
 	#endregion

src/FirebirdSql.Data.FirebirdClient/Common/DbField.cs

@@ -327,9 +327,9 @@ public void SetValue(byte[] buffer)
 							var s = Charset.GetString(buffer, 0, buffer.Length);
 							if((Length % Charset.BytesPerCharacter) == 0)
 							{ 
-								var runes = s.CountRunes();
-								if(runes > CharCount) {
-									s = new string(s.TruncateStringToRuneCount(CharCount));
+								var truncated = s.TruncateStringToRuneCount(CharCount);
+								if(s.AsSpan() != truncated) {
+									s = new string(truncated);
 								}
 							}