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

@@ -68,6 +68,8 @@ int lib_checkbase(int base, FAR const char **pptr)
        */
 
       if (*ptr == '0')
+        {
+          if (ptr[1] != '\0')
             {
               /* Assume octal */
 
@@ -84,6 +86,7 @@ int lib_checkbase(int base, FAR const char **pptr)
                 }
             }
         }
+    }
 
   /* If it a hexadecimal representation,
    * than discard any leading "0X" or "0x"