Fix sign mismatch comparison warnings

Fixes:
rdppm.c:257:14: warning: comparison of integers of different signs: 'int' and 'unsigned int' [-Wsign-compare]
    if (temp > maxval)
        ~~~~ ^ ~~~~~~
rdppm.c:284:14: warning: comparison of integers of different signs: 'int' and 'unsigned int' [-Wsign-compare]
    if (temp > maxval)
        ~~~~ ^ ~~~~~~
rdppm.c:289:14: warning: comparison of integers of different signs: 'int' and 'unsigned int' [-Wsign-compare]
    if (temp > maxval)
        ~~~~ ^ ~~~~~~
rdppm.c:294:14: warning: comparison of integers of different signs: 'int' and 'unsigned int' [-Wsign-compare]
    if (temp > maxval)

rdppm.c

@@ -251,7 +251,7 @@ get_word_gray_row (j_compress_ptr cinfo, cjpeg_source_ptr sinfo)
   ptr = source->pub.buffer[0];
   bufferptr = source->iobuffer;
   for (col = cinfo->image_width; col > 0; col--) {
-    register int temp;
+    register unsigned int temp;
     temp  = UCH(*bufferptr++) << 8;
     temp |= UCH(*bufferptr++);
     if (temp > maxval)
@@ -278,7 +278,7 @@ get_word_rgb_row (j_compress_ptr cinfo, cjpeg_source_ptr sinfo)
   ptr = source->pub.buffer[0];
   bufferptr = source->iobuffer;
   for (col = cinfo->image_width; col > 0; col--) {
-    register int temp;
+    register unsigned int temp;
     temp  = UCH(*bufferptr++) << 8;
     temp |= UCH(*bufferptr++);
     if (temp > maxval)