st/mesa: Validate the result of pipe_transfer_map in make_texture (v2)

When using Freecad, I was getting intermittent segfaults inside of
mesa.  I traced it down to this path in st_cb_drawpixels.c where the
result of pipe_transfer_map wasn't being checked.  In my case, it was
returning NULL because nouveau_bo_new returned ENOENT.  I'm by no
means a mesa developer, but this patch solves the problem for me and
seems reasonable enough.

v2: Marek - also unmap the PBO and release the texture, and call
    the make_texture function sooner for less cleanup

Cc: 18.1 18.2 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 936e0dcd61)

src/mesa/state_tracker/st_cb_drawpixels.c

@@ -566,7 +566,11 @@ make_texture(struct st_context *st,
       dest = pipe_transfer_map(pipe, pt, 0, 0,
                                PIPE_TRANSFER_WRITE, 0, 0,
                                width, height, &transfer);
-
+      if (!dest) {
+         pipe_resource_reference(&pt, NULL);
+         _mesa_unmap_pbo_source(ctx, unpack);
+         return NULL;
+      }
 
       /* Put image into texture transfer.
        * Note that the image is actually going to be upside down in
@@ -1173,6 +1177,13 @@ st_DrawPixels(struct gl_context *ctx, GLint x, GLint y,
       return;
    }
 
+   /* Put glDrawPixels image into a texture */
+   pt = make_texture(st, width, height, format, type, unpack, pixels);
+   if (!pt) {
+      _mesa_error(ctx, GL_OUT_OF_MEMORY, "glDrawPixels");
+      return;
+   }
+
    /*
     * Get vertex/fragment shaders
     */
@@ -1199,13 +1210,6 @@ st_DrawPixels(struct gl_context *ctx, GLint x, GLint y,
       st_upload_constants(st, &st->fp->Base);
    }
 
-   /* Put glDrawPixels image into a texture */
-   pt = make_texture(st, width, height, format, type, unpack, pixels);
-   if (!pt) {
-      _mesa_error(ctx, GL_OUT_OF_MEMORY, "glDrawPixels");
-      return;
-   }
-
    /* create sampler view for the image */
    sv[0] = st_create_texture_sampler_view(st->pipe, pt);
    if (!sv[0]) {