Fix divide-by-zero in program cache tracking

src/gpu/gl/GrGpuGL_program.cpp

Index: src/gpu/gl/GrGpuGL_program.cpp
===================================================================
--- src/gpu/gl/GrGpuGL_program.cpp	(revision 8271)
+++ src/gpu/gl/GrGpuGL_program.cpp	(working copy)
@@ -33,7 +33,8 @@
     SkDebugf("--- Program Cache ---\n");
     SkDebugf("Total requests: %d\n", fTotalRequests);
     SkDebugf("Cache misses: %d\n", fCacheMisses);
-    SkDebugf("Cache miss %%: %f\n", (float)fCacheMisses/(float)fTotalRequests);
+    SkDebugf("Cache miss %%: %f\n", (fTotalRequests > 0) 
+                                    ? (float)fCacheMisses/(float)fTotalRequests : 0.0f);
     SkDebugf("---------------------\n");
 #endif
 }