ss_convert_to_wide returns non NUL-terminated SIZED_STRING when input is empty

Hi, the function `ss_convert_to_wide` does not append a null terminator to the `c_str`. The docs specify the string should be null terminated along with the size: https://github.com/VirusTotal/yara/blob/0f0c667c054979dafeca43149930eaf0e67b0dbb/libyara/include/yara/sizedstr.h#L50-L52 As a result, this causes use of uninitialized memory and/or overflows in downstream code. Tested on the most recent commit `0f0c667c`. (found via automated fuzzing) The following testcase demonstrates the issue. (the second `abort()` triggers) ```cpp #include <cstdio> #include <cstdlib> #include <cstdint> #include <cstring> extern "C" { #include "yara/sizedstr.h" } int main(){ // Create an empty SIZED_STRING SIZED_STRING* s = ss_new(""); if(!s) return 0; // Convert to wide SIZED_STRING* w = ss_convert_to_wide(s); if(!w) return 0; // Expect NUL at position length for both if (s->c_string[s->length] != '\0') abort(); if (w->c_string[w->length] != '\0') abort(); // Fails: w->length==0 but w->c_string[0] != '\0' return 0; } ```