remoteproc: imx_rproc: Skip over memory region when node value is NULL

In the Linux kernel, the following vulnerability has been resolved:

remoteproc: imx_rproc: Skip over memory region when node value is NULL

In imx_rproc_addr_init() "nph = of_count_phandle_with_args()" just counts

number of phandles. But phandles may be empty. So of_parse_phandle() in

the parsing loop (0 < a < nph) may return NULL which is later dereferenced.

Adjust this issue by adding NULL-return check.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

[Fixed title to fit within the prescribed 70-75 charcters]