fix utf8_decode failing on 16-bit codepoints at end of string

[jcl1234]

utf8_decode's 16-bit case checks for max >= 3 rather than max >= 2, which means the sequence will fail to decode if max = 2 (E.g. when the sequence is the last in a string).

Example:

// 'å' & 'ñ' are each encoded in utf8 using a 16-bit sequence.
// str16_from_8 calls utf8_decode internally.
Temp scratch = scratch_begin(0, 0);
String16 result = str16_from_8(scratch.arena, str8_lit("å ñ"));

Expected result:

{
  .str = {0x00e5, 0x0020, 0x00f1}, // "å ñ"
  .size = 3
}

Actual result:

{
  .str = {0x00e5, 0x0020, 0x003f, 0x003f}, // "å ??"
  .size = 4
}