add missing call to va_end() for copied va_list

The man page stdarg(3) states that each invocation of `va_copy()` must
be matched by a corresponding invocation of `va_end()` in the same
function.

Signed-off-by: Max <max@mxzero.net>
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/strings.c

@@ -85,6 +85,7 @@ int string_vcatf(struct string *sb, const char *format, va_list ap)
 			      ap2);
 	}
 	sb->len += n;
+	va_end(ap2);
 
 	return n;
 }