Misc: Don't run etc1 bench with invalid image and fix VS2015 build warning

tools/sk_tool_utils.cpp

Index: tools/sk_tool_utils.cpp
diff --git a/tools/sk_tool_utils.cpp b/tools/sk_tool_utils.cpp
index 9a667b4a45d0d61779854723b88bf54aa34b546e..e2bc0fc42cfb17e284335972234d0c41a043eb18 100644
--- a/tools/sk_tool_utils.cpp
+++ b/tools/sk_tool_utils.cpp
@@ -352,7 +352,7 @@ void make_big_path(SkPath& path) {
 static float gaussian2d_value(int x, int y, float sigma) {
     // don't bother with the scale term since we're just going to normalize the
     // kernel anyways
-    float temp = exp(-(x*x + y*y)/(2*sigma*sigma));
+    float temp = expf(-(x*x + y*y)/(2*sigma*sigma));
     return temp;
 }