[pattern] Silence compiler with impossible case.

Assert that the pattern is one of the four known types, and return an
error so that the compiler knows that the local variable can not be used
uninitialised.

src/cairo-pattern.c

@@ -272,6 +272,9 @@ _cairo_pattern_create_copy (cairo_pattern_t	  **pattern_out,
     case CAIRO_PATTERN_TYPE_RADIAL:
 	pattern = malloc (sizeof (cairo_radial_pattern_t));
 	break;
+    default:
+	ASSERT_NOT_REACHED;
+	return _cairo_error (CAIRO_STATUS_PATTERN_TYPE_MISMATCH);
     }
     if (unlikely (pattern == NULL))
 	return _cairo_error (CAIRO_STATUS_NO_MEMORY);