Avoid segfaults for empty param string

src/navier-stokes.c

@@ -168,6 +168,7 @@ int eea(
 
       // params
       // allocate buffer for params
+      if(params_string!=NULL) {
 	char* params=calloc(sizeof(char), strlen(params_string)+1);
 	strcpy(params, params_string);
 	remove_entry(params, "starting_time");
@@ -175,6 +176,7 @@ int eea(
 	remove_entry(params, "nsteps");
 	fprintf(savefile," -p \"%s;starting_time=%lu;nsteps=%lu;init=file:%s\"", params, t+1, (nsteps == 0 ? 0 : nsteps-t-1), savefile_string);
 	free(params);
+      }
 
       fprintf(savefile," energy\n");