Use Encoding.UTF8 overload accepting span

src/ImageSharp/MetaData/Profiles/Exif/ExifReader.cs

@@ -140,26 +140,27 @@ namespace SixLabors.ImageSharp.MetaData.Profiles.Exif
 
         private unsafe string ConvertToString(ReadOnlySpan<byte> buffer)
         {
-#if NETSTANDARD1_1
+            Span<byte> nullChar = stackalloc byte[1] { 0 };
-            byte[] bytes = buffer.ToArray();
 
-            string result = Encoding.UTF8.GetString(bytes, 0, buffer.Length);
+            int nullCharIndex = buffer.IndexOf(nullChar);
 
+            if (nullCharIndex > -1)
+            {
+                buffer = buffer.Slice(0, nullCharIndex);
+            }
+
+#if NETSTANDARD1_1
+            return Encoding.UTF8.GetString(buffer.ToArray(), 0, buffer.Length);
+#elif NETCOREAPP2_1
+            return Encoding.UTF8.GetString(buffer);
 #else
             string result;
 
             fixed (byte* pointer = &MemoryMarshal.GetReference(buffer))
             {
-                result = Encoding.UTF8.GetString(pointer, buffer.Length);
+                return Encoding.UTF8.GetString(pointer, buffer.Length);
             }
 #endif
-            int nullCharIndex = result.IndexOf('\0');
-            if (nullCharIndex != -1)
-            {
-                result = result.Substring(0, nullCharIndex);
-            }
-
-            return result;
         }
 
         /// <summary>