Fix fb variable name in GLSL

src/gpu/glsl/GrGLSLFragmentShaderBuilder.cpp

Index: src/gpu/glsl/GrGLSLFragmentShaderBuilder.cpp
diff --git a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.cpp b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.cpp
index 4513a69fc85bbab46ac7c306ca7beef50c1c8050..7763f86e42957b664c027a2931a0f8d3da44e807 100644
--- a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.cpp
+++ b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.cpp
@@ -266,6 +266,8 @@ const char* GrGLSLFragmentShaderBuilder::dstColor() {
             fbFetchColorName = DeclaredColorOutputName();
             // Set the dstColor to an intermediate variable so we don't override it with the output
             this->codeAppendf("vec4 %s = %s;", kDstColorName, fbFetchColorName);
+        } else {
+            return fbFetchColorName;
         }
     }
     return kDstColorName;