Fix undefined behaviour warning in kstring

Fixes a "runtime error: applying zero offset to null pointer"
warning from clang 10.0.1 undefined behaviour sanitizer.

This happened when the storage for the string had not been
allocated before calling kvsprintf().  Fix by making it allocate
a buffer if this is the case, so vsnprintf() has something to
write into.

kstring.c

@@ -1,7 +1,7 @@
 /* The MIT License
 
    Copyright (C) 2011 by Attractive Chaos <attractor@live.co.uk>
-   Copyright (C) 2013-2018, 2020 Genome Research Ltd.
+   Copyright (C) 2013-2018, 2020-2021 Genome Research Ltd.
 
    Permission is hereby granted, free of charge, to any person obtaining
    a copy of this software and associated documentation files (the
@@ -153,6 +153,15 @@ int kvsprintf(kstring_t *s, const char *fmt, va_list ap)
 		return l;
 	}
 
+	if (!s->s) {
+		const size_t sz = 64;
+		s->s = malloc(sz);
+		if (!s->s)
+			return -1;
+		s->m = sz;
+		s->l = 0;
+	}
+
 	l = vsnprintf(s->s + s->l, s->m - s->l, fmt, args); // This line does not work with glibc 2.0. See `man snprintf'.
 	va_end(args);
 	if (l + 1 > s->m - s->l) {