Texture2D.CreateExternalTexture is being used incorrectly. CreateExternalTexture() requires the user to pass in a native texture object. When using Direct3D11 for example, this could be a ID3D11Texture2D* texture. The code provided passes in a pointer to raw pixel data in memory. Unity cannot make the distinction between a valid and invalid pointer being passed in. To render a bitmap onto a RenderTexture, this example code would work:

System.Drawing.Imaging.BitmapData bd =
bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height),
System.Drawing.Imaging.ImageLockMode.ReadOnly,
System.Drawing.Imaging.PixelFormat.Format32bppArgb);

int pixelSize = 4; // sizeof(ARGB32)
byte[] data = new byte[bmp.Width * bmp.Height * pixelSize];

Debug.Assert(bd.Stride == bd.Width * pixelSize);
Marshal.Copy(bd.Scan0, data, 0, bmp.Width * bmp.Height * pixelSize);

Texture2D tmptex = new Texture2D(bmp.Width, bmp.Height, TextureFormat.BGRA32, false);
tmptex.LoadRawTextureData(data);
tmptex.Apply();

bmp.UnlockBits(bd);

RenderTexture.active = texture;
UnityEngine.Graphics.Blit(tmptex, texture);

Object.DestroyImmediate(tmptex);