[MIPS] get_frame_info(): null function size means size is unknown

This patch adds 2 sanity checks.

The first one test that the start address of the function to analyze has been
set by the caller. If not return an error since nothing usefull can be done
without.

The second one checks that the function's size has been set. A null size can
happen if CONFIG_KALLSYMS is not set and it means that we don't know the size
of the function to analyze. In this case, we make it equal to 128 instructions
by default.

Signed-off-by: Franck Bui-Huu <vagabon.xyz@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>