h264: Fix parameters to ff_er_add_slice() call

s->mb_x is reset to zero a couple of lines above. It does not make
sense to call ff_er_add_slice() with 0 as endx when the end of the
macroblock row was reached. Fixes unnecessary and counterproductive
error resilience in https://bugzilla.libav.org/show_bug.cgi?id=394.

CC: libav-stable@libav.org