PVS-Studio warning V512 for Virtualdub

bool VDOpenGLBinding::Attach(....) {
  ....
  if (!memcmp(start, "GL_EXT_blend_subtract", 20))
  ....
}

strlen("GL_EXT_blend_subtract") == 21

This suspicious code was found in Virtualdub project by PVS-Studio static code analyzer.
Warning message is:
V512 A call of the 'memcmp' function will lead to underflow of the buffer '"GL_EXT_blend_subtract"'. Riza opengl.cpp 393

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/