bs_read_ue() can return values above INT32_MAX. bs_read_se() stored the
result in an int32_t and computed (r+1)/2, which is undefined behavior
when the code is 0x7FFFFFFF and gives wrong values for codes above
INT32_MAX. The SPS being parsed comes from the host during the H.264
SPS fixup.
Do the conversion in unsigned arithmetic. bs_read_ue() returns at most
0xFFFFFFFE, so both branches produce in-range int32_t values, and every
code that was previously well-defined decodes to the same value.
