Some more trivial source typos

Found via `codespell` and `grep`

contrib/tools/pngcp.c

@@ -2220,7 +2220,7 @@ cp_one_file(struct display *dp, const char *filename, const char *destname)
    strcpy(dp->best, dp->curr);
    dp->best_size = dp->write_size;
 
-   if (dp->nsp > 0) /* interating over lists */
+   if (dp->nsp > 0) /* iterating over lists */
    {
       char *tmpname, tmpbuf[(sizeof dp->namebuf) + 4];
       assert(dp->curr[0] == ' ' && dp->tsp > 0);

contrib/tools/pngfix.c

@@ -772,7 +772,7 @@ skip_chunk_type(const struct global *global, png_uint_32 type)
          return 0;
 
       /* Chunks that specify gamma encoding which should therefore only be
-       * removed the the user insists:
+       * removed if the user insists:
        */
       case png_gAMA: case png_sRGB:
          if (global->skip >= SKIP_ALL)
@@ -3152,7 +3152,7 @@ read_chunk(struct file *file)
       }
    }
 
-   /* Control gets to here if the the stream seems invalid or damaged in some
+   /* Control gets to here if the stream seems invalid or damaged in some
     * way.  Either there was a problem reading all the expected data (this
     * chunk's data, its CRC and the length and type of the next chunk) or the
     * next chunk length/type are invalid.  Notice that the cases that end up
@@ -3710,7 +3710,7 @@ usage(const char *prog)
 "            practice most programs will ignore it.",
 "        bKGD [transform]: This is used by libpng transforms."
 "    --max=<number>:",
-"      Use IDAT chunks sized <number>.  If no number is given the the IDAT",
+"      Use IDAT chunks sized <number>.  If no number is given the IDAT",
 "      chunks will be the maximum size permitted; 2^31-1 bytes.  If the option",
 "      is omitted the original chunk sizes will not be changed.  When the",
 "      option is given --strip=unsafe is set automatically. This may be",