Fix uninit texture key with odd number of textures
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1575993003

Review URL: https://codereview.chromium.org/1575993003
diff --git a/src/gpu/gl/GrGLProgramDesc.cpp b/src/gpu/gl/GrGLProgramDesc.cpp
index 0325625..45e01e7 100644
--- a/src/gpu/gl/GrGLProgramDesc.cpp
+++ b/src/gpu/gl/GrGLProgramDesc.cpp
@@ -30,6 +30,10 @@
         k16[i] = caps.configTextureSwizzle(access.getTexture()->config()).asKey() |
                  (isExternal ? 0xFF00 : 0x0000);
     }
+    // zero the last 16 bits if the number of textures is odd.
+    if (numTextures & 0x1) {
+        k16[numTextures] = 0;
+    }
 }
 
 /**