Initialize variable to appease compiler warning

TBR=bsalomon@google.com
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1854773002

Review URL: https://codereview.chromium.org/1854773002
diff --git a/src/gpu/vk/GrVkPipelineStateBuilder.cpp b/src/gpu/vk/GrVkPipelineStateBuilder.cpp
index 00e81d4..6d46ea9 100644
--- a/src/gpu/vk/GrVkPipelineStateBuilder.cpp
+++ b/src/gpu/vk/GrVkPipelineStateBuilder.cpp
@@ -99,7 +99,7 @@
     moduleCreateInfo.pNext = nullptr;
     moduleCreateInfo.flags = 0;
 
-    shaderc_compilation_result_t result;
+    shaderc_compilation_result_t result = nullptr;
 
     if (gpu->vkCaps().canUseGLSLForShaderModule()) {
         moduleCreateInfo.codeSize = strlen(shaderString.c_str());