st/wgl: fix locking issue in stw_st_framebuffer_present_locked()

st/wgl: fix locking issue in stw_st_framebuffer_present_locked()

When stw_st_framebuffer_present_locked() is called, the
stw_framebuffer's mutex will already be locked.  Normally, the
stw_framebuffer_present_locked() function calls
stw_framebuffer_release() to unlock the mutex when it's done.  But if
for some reason the 'resource' pointer in
stw_st_framebuffer_present_locked() is null, we'd return without
unlocking the stw_framebuffer.  This fixes that to avoid potential
deadlocks.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>