Avoid possible overflow (src/port/bsearch_arg.c)

Hi.

The port function *bsearch_arg* mimics the C function
*bsearch*.

The API signature is:
void *
bsearch_arg(const void *key, const void *base0,
size_t nmemb, size_t size,
int (*compar) (const void *, const void *, void *),
void *arg)

So, the parameter *nmemb* is size_t.
Therefore, a call with nmemb greater than INT_MAX is possible.

Internally the code uses the *int* type to iterate through the number of
members, which makes overflow possible.

Trivial fix attached.

best regards,
Ranier Vilela