Fix memory corruption in mkstr caused by integer overflow

The static 'slot' variable in mkstr() was previously a signed 32-bit
integer. On high-load systems (e.g., during status data dumps), this
counter could overflow, becoming negative.

In C, a negative dividend with the modulo operator (slot % 256) results
in a negative index. This caused the 'ret' pointer to point to memory
addresses BEFORE the actual buffer. In our case, this led to a SIGSEGV
because a timestamp string was written directly over the 'contact_list'
pointer, which happened to be located in that memory region.

Changes:
- Changed 'slot' from signed int to unsigned int (uint) to ensure
  the modulo result is always positive and within buffer bounds.
- This ensures that upon reaching UINT_MAX, the counter wraps
  safely back to zero.

Signed-off-by: nook24 <d.ziegler@avendis.com>

Author: nook24

NEWS

@@ -5,6 +5,9 @@ Breaking changes:
 Features:
 * new attribute 'check_timeout' on `host` and `service` structs, overrides the global host / service check timeouts. (#525)
 
+Bugfixes:
+* fix memory corruption in `mkstr` caused by integer overflow (#527)
+
 1.5.0 - Feb 03 2026
 ===================
 Breaking changes:

lib/nsutils.c

@@ -94,7 +94,7 @@ int str2timeval(char *str, struct timeval *tv)
 const char *mkstr(const char *fmt, ...)
 {
 	static char buf[MKSTR_BUFS][32]; /* 8k statically on the stack */
-	static int slot = 0;
+	static unsigned int slot = 0;
 	char *ret;
 
 	va_list ap;