opengles emulator: fix glGetShaderSource

added NullAllowed flag to length parameter of glGetShaderSource
calling glGetShaderSource with NULL length caused length data
from the host get left on the stream and get the stream out of sync

Change-Id: I45e8e523a67698c9b0cd003e2af7e15c76ed22b7
diff --git a/tools/emulator/opengl/system/GLESv2_enc/gl2.attrib b/tools/emulator/opengl/system/GLESv2_enc/gl2.attrib
index bf266df..e764c10 100644
--- a/tools/emulator/opengl/system/GLESv2_enc/gl2.attrib
+++ b/tools/emulator/opengl/system/GLESv2_enc/gl2.attrib
@@ -175,6 +175,7 @@
 glGetShaderSource
 	dir length out
 	len length (sizeof(GLsizei))
+	var_flag length nullAllowed
 	dir source out
 	len source bufsize