fp_ident: fix incorrect length in strncat() call.

strncat' last argument is the remaining size of the buffer given in the
first argument, not the total buffer length.

Reported-by: contact@oppida.fr via clamav #11166
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>

src/misc/fp_ident.c

@@ -74,7 +74,7 @@ const char *fp_ident(void)
 
    if (sizeof(fp_digit) == sizeof(fp_word)) {
       strncat(buf, "WARNING: sizeof(fp_digit) == sizeof(fp_word), this build is likely to not work properly.\n",
-              sizeof(buf)-1);
+              sizeof(buf) - strlen(buf) - 1);
    }
    return buf;
 }