Avoid some conversion warnings in gzread.c and gzwrite.c.

gzread.c

@@ -314,9 +314,9 @@ local z_size_t gz_read(state, buf, len)
     got = 0;
     do {
         /* set n to the maximum amount of len that fits in an unsigned int */
-        n = -1;
+        n = (unsigned)-1;
         if (n > len)
-            n = len;
+            n = (unsigned)len;
 
         /* first just try copying data from the output buffer */
         if (state->x.have) {
@@ -397,7 +397,7 @@ int ZEXPORT gzread(file, buf, len)
     }
 
     /* read len or fewer bytes to buf */
-    len = gz_read(state, buf, len);
+    len = (unsigned)gz_read(state, buf, len);
 
     /* check for an error */
     if (len == 0 && state->err != Z_OK && state->err != Z_BUF_ERROR)
@@ -447,7 +447,6 @@ z_size_t ZEXPORT gzfread(buf, size, nitems, file)
 int ZEXPORT gzgetc(file)
     gzFile file;
 {
-    int ret;
     unsigned char buf[1];
     gz_statep state;
 
@@ -469,8 +468,7 @@ int ZEXPORT gzgetc(file)
     }
 
     /* nothing there -- try gz_read() */
-    ret = gz_read(state, buf, 1);
-    return ret < 1 ? -1 : buf[0];
+    return gz_read(state, buf, 1) < 1 ? -1 : buf[0];
 }
 
 int ZEXPORT gzgetc_(file)

gzwrite.c

@@ -209,7 +209,7 @@ local z_size_t gz_write(state, buf, len)
                               state->in);
             copy = state->size - have;
             if (copy > len)
-                copy = len;
+                copy = (unsigned)len;
             memcpy(state->in + have, buf, copy);
             state->strm.avail_in += copy;
             state->x.pos += copy;
@@ -229,7 +229,7 @@ local z_size_t gz_write(state, buf, len)
         do {
             unsigned n = (unsigned)-1;
             if (n > len)
-                n = len;
+                n = (unsigned)len;
             state->strm.avail_in = n;
             state->x.pos += n;
             if (gz_comp(state, Z_NO_FLUSH) == -1)