TransformFeedbackTest: fix buffer read overrun

There were 5 floats there, but not 5 vertices (3x5 floats!). This
tripped the AddressSanitizer.

Bug: angleproject:6600
Change-Id: Ie3f96437b087b6ab897fdf451a90e9aac62c2403
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3235905
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
diff --git a/src/tests/gl_tests/TransformFeedbackTest.cpp b/src/tests/gl_tests/TransformFeedbackTest.cpp
index 4c9d6c1..1e69ad6 100644
--- a/src/tests/gl_tests/TransformFeedbackTest.cpp
+++ b/src/tests/gl_tests/TransformFeedbackTest.cpp
@@ -2264,7 +2264,8 @@
     compileDefaultProgram(tfVaryings, GL_INTERLEAVED_ATTRIBS);
 
     GLint positionLocation   = glGetAttribLocation(mProgram, essl1_shaders::PositionAttrib());
-    const GLfloat vertices[] = {-1.0f, -0.5f, 0.0f, 0.5f, 1.0f};
+    const GLfloat vertices[] = {-1.0f, -0.5f, 0.0f,  0.5f, 1.0f,  1.0f, -0.5f, 0.0f,
+                                0.5f,  1.0f,  -1.0f, 0.0f, -0.5f, 0.0f, 1.0f};
 
     glVertexAttribPointer(positionLocation, 3, GL_FLOAT, GL_FALSE, 0, vertices);
     glEnableVertexAttribArray(positionLocation);