shared/libc/string0: Don't deref args for n==0 case.

C99 says that strncmp has UB for either string being NULL, so the
current behavior is technically correct, but it's an easy fix to handle
this case correctly.

7.1.4: "unless explicitly stated otherwise in the detailed
description... if an argument to a function has ...null pointer.. the
behavior is undefined".

7.21.1: "Unless explicitly stated otherwise in the description of a
particular function in this subclause, pointer arguments on such a call
shall still have valid values, as described in 7.1.4".

Also make the same change for the minimal version in bare-arm/lib.c.

This work was funded through GitHub Sponsors.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

ports/bare-arm/lib.c

@@ -110,8 +110,9 @@ char *strchr(const char *s, int c) {
 }
 
 int strncmp(const char *s1, const char *s2, size_t n) {
-    while (*s1 && *s2 && n-- > 0) {
+    while (n > 0 && *s1 && *s2) {
         int c = *s1++ - *s2++;
+        --n;
         if (c) {
             return c;
         }

shared/libc/string0.c

@@ -154,7 +154,7 @@ int strcmp(const char *s1, const char *s2) {
 }
 
 int strncmp(const char *s1, const char *s2, size_t n) {
-    while (*s1 && *s2 && n > 0) {
+    while (n > 0 && *s1 && *s2) {
         char c1 = *s1++; // XXX UTF8 get char, next char
         char c2 = *s2++; // XXX UTF8 get char, next char
         n--;