libc/stdlib: Check if there is next char before assuming octal

When *pptr/ptr is "0" and base is 0, lib_checkbase() returns the error base 8(should be 10).
e.g. `strtoul("0", &endptr, 0);`

Signed-off-by: wangjianyu3 <wangjianyu3@xiaomi.com>

libs/libc/stdlib/lib_checkbase.c

@@ -69,18 +69,21 @@ int lib_checkbase(int base, FAR const char **pptr)
 
       if (*ptr == '0')
         {
-          /* Assume octal */
-
-          base = 8;
-          ptr++;
-
-          /* Check for hexadecimal */
-
-          if ((*ptr == 'X' || *ptr == 'x') &&
-              lib_isbasedigit(ptr[1], 16, NULL))
+          if (ptr[1] != '\0')
             {
-              base = 16;
+              /* Assume octal */
+
+              base = 8;
               ptr++;
+
+              /* Check for hexadecimal */
+
+              if ((*ptr == 'X' || *ptr == 'x') &&
+                  lib_isbasedigit(ptr[1], 16, NULL))
+                {
+                  base = 16;
+                  ptr++;
+                }
             }
         }
     }