Fix FramebufferTexture2DMultisampleEXT validation Generate an error when the samples value is negative. Bug: angleproject:406922380 Change-Id: I508f3ddf82e30291fdcc3086ab19ebfa4f84947c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6421716 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>