jscript: Make sure to initialise the string output parameter in run_exec.

dlls/jscript/regexp.c

@@ -3642,6 +3642,9 @@ static HRESULT run_exec(script_ctx_t *ctx, vdisp_t *jsthis, VARIANT *arg, jsexce
         SysFreeString(string);
         set_last_index(regexp, 0);
         *ret = VARIANT_FALSE;
+        if(input) {
+            *input = NULL;
+        }
         return S_OK;
     }