genksyms: Fix segfault with invalid declarations

Do not try to recover too early and segfault when parsing invalid
declarations such as

echo 'int (int);' | scripts/genksyms/genksyms
echo 'int a, (int);' | scripts/genksyms/genksyms
echo 'extern void *__inline_memcpy((void *), (const void *), (__kernel_size_t));' | scripts/genksyms/genksyms

The last one was a real-life bug with
include/asm-generic/asm-prototypes.h on x86_64.

Reported-and-tested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Michal Marek <mmarek@suse.com>