More efficient long conversion for negative ints in VarIntCodec

[aartigao]

Two bitwise operations can be squashed into a single one.

Paranoid extra mode on, here's the bytecode before/after:

Before

public long scodec$codecs$VarIntCodec$$$toPositiveLong(int);
  Code:
     0: iload_1
     1: iconst_0
     2: if_icmplt     8
     5: iload_1
     6: i2l
     7: lreturn
     8: iload_1
     9: ldc           #21                 // int 2147483647
    11: iand
    12: i2l
    13: getstatic     #23                 // Field NegativeIntSignBit:J
    16: lor
    17: lreturn

After

public long scodec$codecs$VarIntCodec$$$toPositiveLong(int);
  Code:
     0: iload_1
     1: iconst_0
     2: if_icmplt     8
     5: iload_1
     6: i2l
     7: lreturn
     8: iload_1
     9: i2l
    10: ldc2_w        #29                 // long 4294967295l
    13: land
    14: lreturn