core, pulse, modules: Fix undefined behavior with array subscript of invalid type

From the NetBSD manual:

     The first argument of these functions is of type int, but only a very
     restricted subset of values are actually valid.  The argument must either
     be the value of the macro EOF (which has a negative value), or must be a
     non-negative value within the range representable as unsigned char.
     Passing invalid values leads to undefined behavior.

     --  ctype(3)

src/modules/dbus/iface-core.c

@@ -1442,7 +1442,7 @@ static bool contains_space(const char *string) {
     pa_assert(string);
 
     for (p = string; *p; ++p) {
-        if (isspace(*p))
+        if (isspace((unsigned char)*p))
             return true;
     }