st/mesa: don't cast the incomplete framebufer to st_framebuffer

st/mesa: don't cast the incomplete framebufer to st_framebuffer

The incomplete framebuffer is set for a surfaceless context. This leads to
the following error in piglit spec@egl_khr_surfaceless_context@viewport:

==26703==ERROR: AddressSanitizer: global-buffer-overflow on address 0x7f6886e43240 at pc 0x7f68854db0fd bp 0x7ffca404b3b0 sp 0x7ffca404b3a0
READ of size 8 at 0x7f6886e43240 thread T0
    #0 0x7f68854db0fc in st_viewport ../../../mesa-src/src/mesa/state_tracker/st_cb_viewport.c:57
    #1 0x556840176cdb in main tests/egl/spec/egl_khr_surfaceless_context/viewport.c:101
    #2 0x7f688edcf3f0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x203f0)
    #3 0x556840176e19 in _start (/home/nha/amd/piglit/bin/egl-surfaceless-context-viewport+0xe19)

0x7f6886e43240 is located 32 bytes to the left of global variable 'DummyRenderbuffer' defined in '../../../mesa-src/src/mesa/main/fbobject.c:69:31' (0x7f6886e43260) of size 112
0x7f6886e43240 is located 8 bytes to the right of global variable 'IncompleteFramebuffer' defined in '../../../mesa-src/src/mesa/main/fbobject.c:73:30' (0x7f6886e42de0) of size 1112
SUMMARY: AddressSanitizer: global-buffer-overflow ../../../mesa-src/src/mesa/state_tracker/st_cb_viewport.c:57 in st_viewport

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Marek Olšák <marek@olsak@amd.com>