Fix possible invalid write and overflow issues in kmemmem, kstrstr and kstrnstr

[daviesrob]

Fixes various issues in the kmemmem() interface and its
kstrstr() and kstrnstr() wrappers:

• Calling kmemmem(), kstrstr() or kstrnstr() with a zero-length pattern could lead to a zero-length allocation in the ksBM_prep() function and, if that returns a non-NULL suff pointer, an invalid write to suff[-1]. Thanks to Harrison Green for reporting this.

  Fixed by catching zero-length patterns and returning the correct result - a pointer to the start of the input.

  As an additional optimisation, patterns of length 1 are now passed to memchr() as it's likely to be more efficient in that case.

  Finally if the pattern is longer that the input, NULL is returned early as it can never match. This is also useful for some of the other changes, as it ensures subtracting the two lengths will never give a negative result.

• The kmemmem() interface uses int for the sizes of the pattern and input buffers, but the kstrstr() wrapper passed in the output of strlen() which could overflow.

  The implementation is renamed and altered to use size_t for the length of the str buffer. kmemmem() is now a simple wrapper around it that handles the possibility of n being negative. The length of the pat buffer is left as an int as otherwise the type and size of the prep array would need to be changed. This would be wasteful as the pattern is very likely to be short.

• It was difficult to tell the difference between the algorithm failing because it couldn't allocate memory, or the pattern simply not being present (both returned NULL).

  This is fixed by providing Karp-Rabin as a backup algorithm. It's fairly simple and doesn't need to allocate any memory so should always work. It's also used to cover the unlikely case of the kstrstr() pattern being longer than INT_MAX.

• kstrnstr() did not check for a NUL in the first n bytes of its input. This could lead to it finding false matches beyond the strict end of its input, or possibly reads of uninitialised memory.

  Fixed by checking for a NUL in the first n bytes and adjusting n if one is found. It also returns NULL if the pattern is longer than the revised n which both avoids unnecessary work and ensures the pattern length <= INT_MAX.

test/test_kstring.c is updated to add some simple kmemmem, kstrstr, and kstrnstr tests. It's also made to include kstring.c directly so that it can unit test non-exported internal functions. This allows some tests for the Karp-Rabin implementation to be added.