Fix signed/unsigned comparisons properly

Apparently, gcc only warns about [u]int16_t comparisons on platforms
where sizeof(int) == sizeof(int16_t).  Seems like integer type promotion
happens here, and gcc only warns when it changes the comparison result.

The maximum buffer length is 1<<16 (when window size is 15), in which case
end takes values >=0x8000, and start between 0 and 0x8000, inclusive.
Which means we really want to compare unsigned pos to signed start
(except when start is 0x8000), so comparing them as either int16_t or
uint16_t would be incorrect.

But the difference between end-1 and start is always strictly less than
1<<widow_size, which makes it positive when interpreted as int16_t.
The only other case in which the difference between pos and start may
reach a positive value on 16-bit platforms is when indexing is used, the
window size is 15 and the backlog is empty, making start 0x8000, and pos
gets the sentinel end-of-list value 0xFFFF from the index.  This case is
handled by changing the sentinel value to 0 when the backlog is not full.
(Updated, see below.)

Conflicts:
	heatshrink_encoder.c

EDIT:
   Since commit 7e818d55 eliminates the backlog full calculations by
initially filling the backlog with zeroes (i.e., the backlog is always
full), the special case sentinel value of 0 is no longer necessary.

Closes #19.

heatshrink_encoder.c

@@ -488,7 +488,7 @@ static uint16_t find_longest_match(heatshrink_encoder *hse, uint16_t start,
     struct hs_index *hsi = HEATSHRINK_ENCODER_INDEX(hse);
     int16_t pos = hsi->index[end];
 
-    while (pos >= start) {
+    while (pos - (int16_t)start >= 0) {
         uint8_t * const pospoint = &buf[pos];
         len = 0;
 
@@ -512,7 +512,7 @@ static uint16_t find_longest_match(heatshrink_encoder *hse, uint16_t start,
         pos = hsi->index[pos];
     }
 #else    
-    for (int16_t pos=end - 1; pos >= (int16_t)start; pos--) {
+    for (int16_t pos=end - 1; pos - (int16_t)start >= 0; pos--) {
         uint8_t * const pospoint = &buf[pos];
         if ((pospoint[match_maxlen] == needlepoint[match_maxlen])
             && (*pospoint == *needlepoint)) {