avoid use of vasnprintf in base/error.c

commit c94f299fc8 uses vasnprintf
which was previously removed from the libheimbase directory in
order to prevent a dependency on libroken.

Replace vasnprintf with snprintf and malloc to avoid the
dependency.

Change-Id: I029e7e6883406ca7311490a3dab4b65cad3ba70b

base/error.c

@@ -92,12 +92,17 @@ heim_error_t
 heim_error_createv(int error_code, const char *fmt, va_list ap)
 {
     heim_error_t e;
-    char *str = NULL;
+    char *str;
     int len;
 
-    len = vasprintf(&str, fmt, ap);
-    if (len < 0 || str == NULL)
+    str = malloc(1024);
+    if (str == NULL)
+        return NULL;
+    len = vsnprintf(str, 1024, fmt, ap);
+    if (len < 0) {
+        free(str);
 	return NULL;
+    }
 
     e = _heim_alloc_object(&_heim_error_object, sizeof(struct heim_error));
     if (e) {