Fixed some warnings on GCC

Mozilla has recently made using NULL as integer an error (via
-Werror=conversion-null GCC flag), which caused ANGLE to no longer compile on
mingw in Mozilla tree. The ones that may not be obvious are removal of some <0
checks. They are not needed because they were done on unsigned types.

Author: Jacek Caban
http://codereview.appspot.com/6679049

(With changes to apply to dx11proto branch)

git-svn-id: https://angleproject.googlecode.com/svn/branches/dx11proto@1464 736b8ea6-26fd-11df-bfd4-992fa37f6226
diff --git a/src/libGLESv2/Shader.cpp b/src/libGLESv2/Shader.cpp
index 1b04864..7051b34 100644
--- a/src/libGLESv2/Shader.cpp
+++ b/src/libGLESv2/Shader.cpp
@@ -315,7 +315,7 @@
 void Shader::compileToHLSL(void *compiler)
 {
     // ensure we don't pass a NULL source to the compiler
-    char *source = "\0";
+    const char *source = "\0";
     if (mSource)
     {
         source = mSource;
@@ -504,7 +504,7 @@
 
     // set by ParseAttributes
     mAttributes.clear();
-};
+}
 
 void VertexShader::compile()
 {