Fix the conversion process from an integer to a hexadecimal string

The original implementation of __str_base16(), which converts
integers to hexadecimal strings, causes errors when processing
negative integers.

It produces redundant 'f' characters at the beginning of the
converted string due to sign extension and results in unexpected
output when calling printf() or sprintf().

Therefore, this commit fixes it and ensure correct conversion.

Author: DrXiao

lib/c.c

@@ -178,7 +178,8 @@ void __str_base8(char *pb, int val)
 void __str_base16(char *pb, int val)
 {
     int c = INT_BUF_LEN - 1;
-    while (c > 0) {
+    int times = sizeof(int) << 1;
+    for (int i = 0; i < times; i++) {
         int v = val & 0xf;
         if (v < 10)
             pb[c] = '0' + v;