Fixes gcc warnings of the form:

IdOfPr.c: In function ‘XcmsFormatOfPrefix’:
IdOfPr.c:69:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if ((len = strlen(prefix)) >= sizeof(string_buf)) {
                                ^
IdOfPr.c:83:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (len >= sizeof(string_buf)) Xfree(string_lowered);
           ^
IdOfPr.c:97:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (len >= sizeof(string_buf)) Xfree(string_lowered);
           ^
IdOfPr.c:104:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if (len >= sizeof(string_buf)) Xfree(string_lowered);
             ^
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Backported-to-NX-by: Ulrich Sibiller <uli42@gmx.de>