nanobench: Add some Valgrind failure paranoia.

Trying to figure out

http://build.chromium.org/p/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-Valgrind/builds/586/steps/nanobench/logs/stdio

BUG=skia:4482

Review URL: https://codereview.chromium.org/1418633002
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index 38f554d..b2a35df 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -1084,6 +1084,7 @@
             log->configOption("clip",
                     SkStringPrintf("%d %d %d %d", fClip.fLeft, fClip.fTop,
                                                   fClip.fRight, fClip.fBottom).c_str());
+            SK_ALWAYSBREAK(fCurrentScale < fScales.count());  // debugging paranoia
             log->configOption("scale", SkStringPrintf("%.2g", fScales[fCurrentScale]).c_str());
             if (fCurrentUseMPD > 0) {
                 SkASSERT(1 == fCurrentUseMPD || 2 == fCurrentUseMPD);