textures: ignore texture 0 in glDeleteTextures()

Ths OpenGL spec says that the texture index 0 must be silently ignored
by glDeleteTextures(). Failure to ignore this value was causing weird
artifacts with text labels in Neverball.

We also invert the logic of the "if" condition, to make it easier to
read.

src/texture.c

@@ -523,7 +523,7 @@ void glDeleteTextures(GLsizei n, const GLuint *textures)
     GX_DrawDone();
     while (n-- > 0) {
         int i = *texlist++;
-        if (!(i < 0 || i >= _MAX_GL_TEX)) {
+        if (i > 0 && i < _MAX_GL_TEX) {
             void *data = GX_GetTexObjData(&texture_list[i].texobj);
             if (data != 0)
                 free(MEM_PHYSICAL_TO_K0(data));