bug (index out of bounds) in MarbleTexture::Evaluate()

[wahn]

Add the following check and you will see that the following lines sometimes produce an index out of bounds:

> git diff
diff --git a/src/textures/marble.h b/src/textures/marble.h
index 26e231b..054d0b2 100644
--- a/src/textures/marble.h
+++ b/src/textures/marble.h
@@ -73,6 +73,7 @@ class MarbleTexture : public Texture<Spectrum> {
 #define NC sizeof(c) / sizeof(c[0])
 #define NSEG (NC - 3)
         int first = std::floor(t * NSEG);
+        CHECK_LT(first, NSEG);
         t = (t * NSEG - first);
         Spectrum c0 = Spectrum::FromRGB(c[first]);
         Spectrum c1 = Spectrum::FromRGB(c[first + 1]);

> ~/builds/pbrt/release/pbrt breakfast-lamps.pbrt
pbrt version 3 (built Apr  1 2019 at 16:44:39) [Detected 8 cores]
Copyright (c)1998-2018 Matt Pharr, Greg Humphreys, and Wenzel Jakob.
The source code to pbrt (but *not* the book contents) is covered by the BSD License.
See the file LICENSE.txt for the conditions of the license.
Rendering: [                                                                   ]  (1.0s|?s)  
F0828 15:33:15.123517 15723 marble.h:76] Check failed: first < (sizeof(c) / sizeof(c[0]) - 3) (6 vs. 6) 
*** Check failure stack trace: ***
    @           0x73618a  google::LogMessage::Fail()
    @           0x736439  google::LogMessage::SendToLog()
    @           0x7352df  google::LogMessage::Flush()
    @           0x73ad69  google::LogMessageFatal::~LogMessageFatal()
    @           0x5f090a  pbrt::MarbleTexture::Evaluate()
    @           0x576c42  pbrt::PlasticMaterial::ComputeScatteringFunctions()
    @           0x4ca084  pbrt::GeometricPrimitive::ComputeScatteringFunctions()
    @           0x6060f8  pbrt::SurfaceInteraction::ComputeScatteringFunctions()
    @           0x5265b7  pbrt::RandomWalk()
    @           0x528114  pbrt::GenerateLightSubpath()
    @           0x52aa17  _ZZN4pbrt14BDPTIntegrator6RenderERKNS_5SceneEENKUlNS_6Point2IiEEE_clES5_
    @           0x4af2d9  pbrt::workerThreadFunc()
    @           0x4afe13  std::thread::_Impl<>::_M_run()
    @     0x7f24214c1360  (unknown)
    @     0x7f2421931e25  start_thread
    @     0x7f2420c25bad  __clone
    @              (nil)  (unknown)
Abort (core dumped)

[mmp]

Fixed--thanks!