tests(strnstr): add test 15. test finding the string when it exists i…

[douglasfanucchi]

The following ft_strnstr implementation passes in those 14 tests but it fails when trying to find a string that is present in the haystack and the n bytes to lookup is a number greater than the length of the haystack.

#include <libft.h>

char *ft_strnstr(const char *haystack, const char *search, size_t n)
{
	size_t	i;
	size_t	j;
	size_t	len;

	i = 0;
	len = ft_strlen(search);
	if (!len && !haystack[0])
		return ((char *)haystack);
	while (i < n && haystack[i])
	{
		j = 0;
		while (i + j < n && haystack[i + j] == search[j])
			j++;
		if (j == len)
			return ((char *)haystack + i);
		i++;
	}
	return ((void *)0);
}

The test added in the pull request detects this problem in the above implementation.

[douglasfanucchi]

I just found other test cases that detects problems in that implementation, I'll be adding them to this PR later.