Fix non '\0' terminated labels for InputText

Previously only the length of the label was allocated. As .net is
initializing arrays with 0 and (at least for this stackalloc here)
this happens in 4-byte chunks only strings with a length multiple of 4
were affected as in this case the allocated size exactly matched the
length of the string and so no 0 was left to be used for string end.
Now the array is generated always one byte larger than the label length,
so there will always be a 0 at the end.

src/ImGui.NET/ImGui.Manual.cs

@@ -44,7 +44,7 @@ namespace ImGuiNET
         {
 
             int labelByteCount = Encoding.UTF8.GetByteCount(label);
-            byte* labelBytes = stackalloc byte[labelByteCount];
+            byte* labelBytes = stackalloc byte[labelByteCount + 1];
             fixed (char* labelPtr = label)
             {
                 Encoding.UTF8.GetBytes(labelPtr, label.Length, labelBytes, labelByteCount);
@@ -83,7 +83,7 @@ namespace ImGuiNET
             IntPtr user_data)
         {
             int labelByteCount = Encoding.UTF8.GetByteCount(label);
-            byte* labelBytes = stackalloc byte[labelByteCount];
+            byte* labelBytes = stackalloc byte[labelByteCount + 1];
             fixed (char* labelPtr = label)
             {
                 Encoding.UTF8.GetBytes(labelPtr, label.Length, labelBytes, labelByteCount);
@@ -146,7 +146,7 @@ namespace ImGuiNET
             IntPtr user_data)
         {
             int labelByteCount = Encoding.UTF8.GetByteCount(label);
-            byte* labelBytes = stackalloc byte[labelByteCount];
+            byte* labelBytes = stackalloc byte[labelByteCount + 1];
             fixed (char* labelPtr = label)
             {
                 Encoding.UTF8.GetBytes(labelPtr, label.Length, labelBytes, labelByteCount);
@@ -216,7 +216,7 @@ namespace ImGuiNET
         {
 
             int labelByteCount = Encoding.UTF8.GetByteCount(label);
-            byte* labelBytes = stackalloc byte[labelByteCount];
+            byte* labelBytes = stackalloc byte[labelByteCount + 1];
             fixed (char* labelPtr = label)
             {
                 Encoding.UTF8.GetBytes(labelPtr, label.Length, labelBytes, labelByteCount);