Fix bug with -cdf on junk crossing buffer boundary.

If the gzip stream ended one or two bytes before the buffer
boundary, the first few bytes of the following junk copied would
be incorrect.

pigz.c

@@ -514,7 +514,7 @@ local struct {
     int force;              // true to overwrite, compress links, cat
     int sync;               // true to flush output file
     int form;               // gzip = 0, zlib = 1, zip = 2 or 3
-    unsigned char magic1;   // first byte of possible header when decoding
+    int magic1;             // first byte of possible header when decoding
     int recurse;            // true to dive down into directory structure
     char *sufx;             // suffix to use (".gz" or user supplied)
     char *name;             // name for gzip or zip header
@@ -2570,7 +2570,6 @@ local void in_init(void) {
 // Buffered reading macros for decompression and listing.
 #define GET() (g.in_left == 0 && (g.in_eof || load() == 0) ? 0 : \
                (g.in_left--, *g.in_next++))
-#define UNGET(n) (g.in_left += (n), g.in_next -= (n))
 #define GET2() (tmp2 = GET(), tmp2 + ((unsigned)(GET()) << 8))
 #define GET4() (tmp4 = GET2(), tmp4 + ((unsigned long)(GET2()) << 16))
 #define SKIP(dist) \
@@ -2712,14 +2711,14 @@ local int get_header(int save) {
     // see if it's a gzip, zlib, or lzw file
     g.form = -1;
     g.magic1 = GET();
-    if (g.in_eof)
+    if (g.in_eof) {
+        g.magic1 = -1;
         return -1;
+    }
     magic = (unsigned)g.magic1 << 8;
     magic += GET();
-    if (g.in_eof) {
-        UNGET(1);
+    if (g.in_eof)
         return -2;
-    }
     if (magic % 31 == 0 && (magic & 0x8f20) == 0x0800) {
         // it's zlib
         g.form = 1;
@@ -2778,7 +2777,8 @@ local int get_header(int save) {
         return g.in_eof ? -3 : (int)method;
     }
     if (magic != 0x1f8b) {          // not gzip
-        UNGET(2);
+        g.in_left++;                // return the second byte
+        g.in_next--;
         return -2;
     }
 
@@ -3075,6 +3075,12 @@ local void list_info(void) {
 // --- copy input to output (when acting like cat) ---
 
 local void cat(void) {
+    // copy the first header byte read, if any
+    if (g.magic1 != -1) {
+        unsigned char buf[1] = {g.magic1};
+        g.out_tot += writen(g.outd, buf, 1);
+    }
+
     // copy the remainder of the input to the output
     while (g.in_left) {
         g.out_tot += writen(g.outd, g.in_next, g.in_left);