The doc says those function are like av_free if size or nmemb is
zero. It doesn't match the code. av_realloc() realloc one byte if
size is zero, which was added by 91ff05f6 ten years ago.
realloc() itself in C is implementation-dependent. Make the doc
match the longstanding behaviour.
Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>