common: fix potential NULL dereference in print_rss_hkey()

Static analyzer (Svace) reported a possible null pointer dereference
in print_rss_hkey(). Specifically, when the 'hkey' pointer is NULL,
the function continues execution after printing an error message,
leading to dereferencing hkey[i].

This patch adds an early return after the NULL check to prevent
execution from continuing in such cases.

This resolves:
DEREF_AFTER_NULL: common.c:209

Found by Svace static analysis tool.

Signed-off-by: Anton Moryakov <ant.v.moryakov@gmail.com>