libs: decoder: h264, h265: fix g_return_val_if_fail() missuse

g_return_val_fail() documentations says:

If expr evaluates to FALSE, the current function should be considered to have undefined behaviour (a programmer error). The only correct solution to such an error is to change the module that is calling the current function, so that it avoids this incorrect call.

So it was missused in a couple parts of the H264 and H265 internal decoders. This patch changes that to plain conditionals.

Also, it was included a couple code-style fixes.