Prevent out of bound writes if a shader has too many stages.

Display the same warning ioq3 and OpenJK display, but don't mark the shader as invalid to prevent it from showing as missing shader for existing maps.

src/renderer/tr_shader.cpp

@@ -2157,6 +2157,12 @@ static qboolean ParseShader( const char **text )
 		// stage definition
 		else if ( token[0] == '{' )
 		{
+			if ( s >= MAX_SHADER_STAGES )
+			{
+				ri.Printf( PRINT_WARNING, "WARNING: too many stages in shader %s (max is %i)\n", shader.name, MAX_SHADER_STAGES );
+				break; // Don't return with qfalse, because we don't want existing shaders violating MAX_SHADER_STAGES to be displayed as missing shaders
+			}
+
 			if ( !ParseStage( &stages[s], text ) )
 			{
 				return qfalse;