Code contains multiple always true byte < checks

[Marcono1234]

Describe the bug
The code contains multiple x < ... checks where x is a byte and the right-hand side is > 127 (= max byte value), so these conditions are always true:

• drill/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/CharSubstring.java

  Line 92 in 5f8cdda

  if (currentByte < 128) {

• drill/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/CharSubstring.java

  Line 116 in 5f8cdda

  if (currentByte < 128) {

• drill/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/CharSubstring.java

  Line 127 in 5f8cdda

  if (currentByte < 128) {

• drill/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/StringFunctions.java

  Line 1111 in 5f8cdda

  if (currentByte < 0x128 || // 1-byte char. First byte is 0xxxxxxx.

• drill/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/StringFunctions.java

  Line 1260 in 5f8cdda

  if (currentByte < 0x128 || // 1-byte char. First byte is 0xxxxxxx.

Not completely sure what the fix here is, maybe (byte & 0xFF) < .... And also for StringFunctions maybe it should be 128 (decimal) instead of 0x128 (hex, = 296 decimal); at least the 0x128 looks a bit suspicious.