Fix case where GrGLEffectMatrix gives back a dangling ptr for the coords var name.

src/effects/SkMagnifierImageFilter.cpp

Index: src/effects/SkMagnifierImageFilter.cpp
diff --git a/src/effects/SkMagnifierImageFilter.cpp b/src/effects/SkMagnifierImageFilter.cpp
index aa91b1c9804e20f68059e6bf125b048d925a6c9b..f6b70fc524cba6e0dc76fb190cf4fcc07ca90bb9 100644
--- a/src/effects/SkMagnifierImageFilter.cpp
+++ b/src/effects/SkMagnifierImageFilter.cpp
@@ -127,7 +127,7 @@ void GrGLMagnifierEffect::emitCode(GrGLShaderBuilder* builder,
                                    const char* outputColor,
                                    const char* inputColor,
                                    const TextureSamplerArray& samplers) {
-    const char* coords;
+    SkString coords;
     fEffectMatrix.emitCodeMakeFSCoords2D(builder, key, &coords);
     fOffsetVar = builder->addUniform(
         GrGLShaderBuilder::kFragment_ShaderType |
@@ -142,10 +142,10 @@ void GrGLMagnifierEffect::emitCode(GrGLShaderBuilder* builder,
         GrGLShaderBuilder::kVertex_ShaderType,
         kVec2f_GrSLType, "uInset");
 
-    builder->fsCodeAppendf("\t\tvec2 coord = %s;\n", coords);
+    builder->fsCodeAppendf("\t\tvec2 coord = %s;\n", coords.c_str());
     builder->fsCodeAppendf("\t\tvec2 zoom_coord = %s + %s / %s;\n",
                            builder->getUniformCStr(fOffsetVar),
-                            coords,
+                           coords.c_str(),
                            builder->getUniformCStr(fZoomVar));
 
     builder->fsCodeAppend("\t\tvec2 delta = min(coord, vec2(1.0, 1.0) - coord);\n");