Improve print parsed (#21790)

* Refactor double formatting in eval-utils.c: replace custom logic with `print_netdata_double` for clarity and maintainability.

* Add null checks for `dst` and `fmt` in `vsnprintfz` to prevent potential issues

* Add null check for `fmt` in `vsnprintfz` and ensure `dst` is properly initialized

Author: stelfrag

src/libnetdata/eval/eval-utils.c

@@ -93,18 +93,8 @@ void print_parsed_as_constant(BUFFER *out, NETDATA_DOUBLE n) {
         return;
     }
 
-    char b[100+1], *s;
-    snprintfz(b, sizeof(b) - 1, NETDATA_DOUBLE_FORMAT, n);
-
-    s = &b[strlen(b) - 1];
-    while(s > b && *s == '0') {
-        *s ='\0';
-        s--;
-    }
-
-    if(s > b && *s == '.')
-        *s = '\0';
-
+    char b[DOUBLE_MAX_LENGTH];
+    print_netdata_double(b, n);
     buffer_strcat(out, b);
 }
 

src/libnetdata/libnetdata.c

@@ -63,7 +63,12 @@ char *fgets_trim_len(char *buf, size_t buf_size, FILE *fp, size_t *len) {
 
 // vsnprintfz() returns the number of bytes actually written - after possible truncation
 int vsnprintfz(char *dst, size_t n, const char *fmt, va_list args) {
-    if(unlikely(!n)) return 0;
+    if(unlikely(!dst || !n)) return 0;
+
+    if(unlikely(!fmt)) {
+        dst[0] = '\0';
+        return 0;
+    }
 
     int size = vsnprintf(dst, n, fmt, args);
     dst[n - 1] = '\0';