Fix a divide-by-zero

Only happened when JIT was enabled, you selected libdvm_assert.so, and
the JIT didn't find anything useful to do before the VM exited.

Change-Id: I031d6c1c589a90f7c4580c262caa9e986cda9e63
diff --git a/vm/interp/Jit.c b/vm/interp/Jit.c
index d5ba07c..7b4f160 100644
--- a/vm/interp/Jit.c
+++ b/vm/interp/Jit.c
@@ -511,6 +511,7 @@
              gDvmJit.invokePolyGetterInlined, gDvmJit.invokePolySetterInlined);
         LOGD("JIT: Total compilation time: %llu ms", gDvmJit.jitTime / 1000);
         LOGD("JIT: Avg unit compilation time: %llu us",
+             gDvmJit.numCompilations == 0 ? 0 :
              gDvmJit.jitTime / gDvmJit.numCompilations);
 #endif