UBSan: Avoid unsigned integer overflow in utf8_{,un}escape()

UBSan: Avoid unsigned integer overflow in utf8_{,un}escape()

Split the if/while loop condition into two independent steps so that
in_size-- happens only in the case in_size is nonzero. This gets rid of
unnecessary UBSan warnings.

common.c:1087:16: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'size_t' (aka 'unsigned long')
common.c:1076:16: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'size_t' (aka 'unsigned long')
common.c:1119:16: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'size_t' (aka 'unsigned long')

Signed-off-by: Jouni Malinen <j@w1.fi>