Fix potential buffer overflow caused by long tcMod args

Found by Coverity.

code/renderergl2/tr_shader.c

@@ -1265,8 +1265,8 @@ static qboolean ParseStage( shaderStage_t *stage, char **text )
 				token = COM_ParseExt( text, qfalse );
 				if ( token[0] == 0 )
 					break;
-				strcat( buffer, token );
-				strcat( buffer, " " );
+				Q_strcat( buffer, sizeof (buffer), token );
+				Q_strcat( buffer, sizeof (buffer), " " );
 			}
 
 			ParseTexMod( buffer, stage );