Reproduction Steps:
1. Open project
2. Build and deploy to iOS device
3. Start App in Portrait mode
4. Rotate phone into Landscape mode
5. Notice that some letters appear on the screen
Note: This is text font texture used for UI buttons.
6. Rotate device back to Portrait mode
7. Notice that texture persist.

Expected behaviour: No random text texture are taken if WebCamTexture is not initialised.
Actual result: WebCamTexture takes random texture in this case UI Button text font texture if it is not initialised.

Reproduced with Metal only.
GLES2 and GLES3 show black texture if WebCamTexture is not initialised.
Reproduced on versions: Unity 5.3.7p3, 5.4.4f1, 5.5.0p3, 5.6.0b3
Reproduced on devices: iPhone 6 Plus iOS 8.0, iPad Mini 3 iOS 9.3.2, iPhone 6 Plus iOS 10.1

Did not reproduce on Android.