mesa: Add condition in glGetTexImage for zero size textures

TestMipMaps() function in src/OGLconform/textureNPOT.c calls glTexImage2D()
with width = 0. Texture with zero size skips miptree allocation due to a
condition in function _mesa_store_teximage3d(). While calling glGetTexImage()
it results in assertion failure in intel_map_texture_image() due to null mt
pointer.

This patch fixes the issue by detecting the zero size texture early in
glGetTexImage and glGetCompressedTexImage functions. In such a case function
simply returns doing nothing.
Verified that below mentioned bug is fixed by this patch.

https://bugs.freedesktop.org/show_bug.cgi?id=42334

NOTE: This is a candidate for stable branches

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/texgetimage.c

@@ -837,6 +837,9 @@ _mesa_GetnTexImageARB( GLenum target, GLint level, GLenum format,
    texObj = _mesa_get_current_tex_object(ctx, target);
    texImage = _mesa_select_tex_image(ctx, texObj, target, level);
 
+   if (_mesa_is_zero_size_texture(texImage))
+      return;
+
    if (MESA_VERBOSE & (VERBOSE_API | VERBOSE_TEXTURE)) {
       _mesa_debug(ctx, "glGetTexImage(tex %u) format = %s, w=%d, h=%d,"
                   " dstFmt=0x%x, dstType=0x%x\n",
@@ -970,6 +973,9 @@ _mesa_GetnCompressedTexImageARB(GLenum target, GLint level, GLsizei bufSize,
    texObj = _mesa_get_current_tex_object(ctx, target);
    texImage = _mesa_select_tex_image(ctx, texObj, target, level);
 
+   if (_mesa_is_zero_size_texture(texImage))
+      return;
+
    if (MESA_VERBOSE & (VERBOSE_API | VERBOSE_TEXTURE)) {
       _mesa_debug(ctx,
                   "glGetCompressedTexImage(tex %u) format = %s, w=%d, h=%d\n",

src/mesa/main/teximage.h

@@ -44,7 +44,14 @@ _mesa_is_cube_face(GLenum target)
            target <= GL_TEXTURE_CUBE_MAP_NEGATIVE_Z_ARB);
 }
 
-
+/** Is any of the dimensions of given texture equal to zero? */
+static inline GLboolean
+_mesa_is_zero_size_texture(const struct gl_texture_image *texImage)
+{
+   return (texImage->Width == 0 ||
+           texImage->Height == 0 ||
+           texImage->Depth == 0);
+}
 
 /** \name Internal functions */
 /*@{*/