[bug fix] khttp_file_size return type is 'size_t' (unsigned int), and it always >= 0, so these condition will always be false
[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]
remove re-define 'KHTTP_PATH_LEN'
Merge pull request #1 from zlargon/develop

fix bug, and compile warning