Use 'snprintf' instead of 'sprintf'

This gets rid of a linker warning when compiling on OpenBSD:

    ./.libs/libjq.a(jv_print.o)(.text+0x2fc): In function `jvp_dump_string':
    /root/jq-1.5/jv_print.c:118: warning: sprintf() is often misused, please use snprintf()

jv_print.c

@@ -115,10 +115,10 @@ static void jvp_dump_string(jv str, int ascii_only, FILE* F, jv* S, int T) {
     }
     if (unicode_escape) {
       if (c <= 0xffff) {
-        sprintf(buf, "\\u%04x", c);
+        snprintf(buf, sizeof(buf), "\\u%04x", c);
       } else {
         c -= 0x10000;
-        sprintf(buf, "\\u%04x\\u%04x",
+        snprintf(buf, sizeof(buf), "\\u%04x\\u%04x",
                 0xD800 | ((c & 0xffc00) >> 10),
                 0xDC00 | (c & 0x003ff));
       }