acrn-hv: code review fix lib/string.c

In lib/string.c, strncmp doesn't consider condition "n_arg=0",
just add a process to "n_arg=0".

Tracked-On: #4093
Tracked-On: #3466
Signed-off-by: YanX Fu <yanx.fu@intel.com>
Reviewed-by: Yonghua Huang <yonghua.huang@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

Author: fuyanX

hypervisor/lib/string.c

@@ -232,19 +232,26 @@ int32_t strcmp(const char *s1_arg, const char *s2_arg)
 	return *str1 - *str2;
 }
 
+/**
+ * @pre n_arg > 0
+ */
 int32_t strncmp(const char *s1_arg, const char *s2_arg, size_t n_arg)
 {
 	const char *str1 = s1_arg;
 	const char *str2 = s2_arg;
 	size_t n = n_arg;
+	int32_t ret = 0;
 
-	while (((n - 1) != 0U) && ((*str1) != '\0') && ((*str2) != '\0') && ((*str1) == (*str2))) {
-		str1++;
-		str2++;
-		n--;
+	if (n > 0U) {
+		while (((n - 1) != 0U) && ((*str1) != '\0') && ((*str2) != '\0') && ((*str1) == (*str2))) {
+			str1++;
+			str2++;
+			n--;
+		}
+		ret = (int32_t) (*str1 - *str2);
 	}
 
-	return *str1 - *str2;
+	return ret;
 }
 
 /*