Assert that no FP needs local coords when the variable is void

Change-Id: Ie7c687649313d047639ed96df765d16d59b58090
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/341796
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>

src/gpu/glsl/GrGLSLGeometryProcessor.cpp

@@ -23,6 +23,9 @@ void GrGLSLGeometryProcessor::emitCode(EmitArgs& args) {
     if (gpArgs.fLocalCoordVar.getType() != kVoid_GrSLType) {
         this->collectTransforms(args.fVertBuilder, args.fVaryingHandler, args.fUniformHandler,
                                 gpArgs.fLocalCoordVar, args.fFPCoordTransformHandler);
+    } else {
+        // Make sure no FPs needed the local coord variable.
+        SkASSERT(!*args.fFPCoordTransformHandler);
     }
 
     if (args.fGP.willUseTessellationShaders()) {