tool_paramhlp: fixed potentially uninitialized strtol() variable

Seems highly unlikely to actually be possible, but better safe than
sorry.

Closes #5417

src/tool_paramhlp.c

@@ -181,6 +181,7 @@ ParameterError str2num(long *val, const char *str)
 {
   if(str) {
     char *endptr;
+    endptr = (void*)0;
     long num;
     errno = 0;
     num = strtol(str, &endptr, 10);