rtmp: fix multiple broken overflow checks

Sanity checks like `data + size >= data_end || data + size < data' are
broken, because `data + size < data' assumes pointer overflow, which is
undefined behavior in C.  Many compilers such as gcc/clang optimize such
checks away.

Use `size < 0 || size >= data_end - data' instead.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Martin Storsjö <martin@martin.st>
