mx_getopt: Remove a NULL pointer check

The only caller of the static find_short_option() function doesn't calls
it with a NULL pointer value as the third argument:

    idx = find_short_option(optctl->options, &optctl->_unhandled_shortopts, &optctl->optarg);

So it is not necessary for the function to check the pointer for NULL.

The disadvantage of doing is, that the llvm static analyzer concludes
from the check, that it was possible that the function would be called
with a NULL pointer and questiones a later access in the same function,
which is unchecked:

    mx_getopt.c:158:16: warning: Dereference of null pointer (loaded from variable 'optarg') [core.NullDereference]
           *optarg = *name;
            ~~~~~~ ^
    1 warning generated.

Remove the check.

mx_getopt.c

@@ -127,8 +127,7 @@ static int find_short_option(struct mx_option *options, char **name, char **opta
 
    assert(short_opt);
 
-   if (optarg)
-       *optarg = NULL;
+   *optarg = NULL;
 
    for (i=0, idx=-1; options[i].long_opt || options[i].short_opt; i++) {