Fix unused-but-set-variable totQuadsSeen

In a Windows component release build with Chromium clang GrTextureOp.cpp
FillInVertices totQuadsSeen is calculated but never used. Put it behind
SkDEBUGCODE to match the other post condition asserts.

Change-Id: I6d26709caa952bdad8744ce5bdd82e11dbfa521e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/433437
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed