Add checks for failed SkImageInfo2GrPixelConfig calls

Bug: skia:7645
Change-Id: I32b9326fb43490ef6a7a99527243b68d89184ae1
Reviewed-on: https://skia-review.googlesource.com/109083
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/tests/ProxyTest.cpp b/tests/ProxyTest.cpp
index 20d721b..f48e2a0 100644
--- a/tests/ProxyTest.cpp
+++ b/tests/ProxyTest.cpp
@@ -199,6 +199,7 @@
         for (auto colorType : { kAlpha_8_SkColorType, kRGBA_8888_SkColorType }) {
             for (auto numSamples : {1, 4}) {
                 GrPixelConfig config = SkImageInfo2GrPixelConfig(colorType, nullptr, caps);
+                SkASSERT(kUnknown_GrPixelConfig != config);
                 int supportedNumSamples = caps.getRenderTargetSampleCount(numSamples, config);
 
                 if (!supportedNumSamples) {