gen_param: replace vsprintf by vsnprintf

src/mpim_grid/grid_proj.cc

@@ -53,12 +53,13 @@ check_xyvals(size_t nvals, double *xvals, double *yvals)
 static std::string
 gen_param(const char *fmt, ...)
 {
-  va_list args;
-  char str[256];
+  constexpr size_t len = 256;
+  char str[len];
 
+  va_list args;
   va_start(args, fmt);
 
-  vsprintf(str, fmt, args);
+  vsnprintf(str, len, fmt, args);
 
   va_end(args);