[bug fix] when malloc failed, return -KHTTP_ERR_OOM
[fix compile warning] type conversion and function declaration

1. khttp_base64_encode()
   passing 'char [1024]' to parameter of type 'const unsigned char *' converts between pointers to integer types with different sign [-Wpointer-sign]

2. include openssl/err.h
   implicit declaration of function 'ERR_error_string_n' is invalid in C99 [-Wimplicit-function-declaration]