Fixed small bug in shader generation

http://codereview.appspot.com/6259053/



git-svn-id: http://skia.googlecode.com/svn/trunk@4064 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/gl/GrGLShaderBuilder.cpp b/src/gpu/gl/GrGLShaderBuilder.cpp
index 289e7be..2e1f48e 100644
--- a/src/gpu/gl/GrGLShaderBuilder.cpp
+++ b/src/gpu/gl/GrGLShaderBuilder.cpp
@@ -116,6 +116,8 @@
 void GrGLShaderBuilder::computeModulate(const char* fsInColor) {
     if (NULL != fsInColor) {
         fModulate.printf(" * %s", fsInColor);
+    } else {
+        fModulate.reset();
     }
 }