mport package bundle downloads allow unsafe destination filenames

mport is the MidnightBSD Package Manager. Prior to 2.7.8, libmport/fetch.c did not consistently reject empty, dot, dot-dot, or slash-containing bundle filenames before composing package download and write paths. Malicious package index data could place an unsafe value in indexEntry->bundlefile, and the missing is_valid_bundle_filename() checks allowed downloaded package data to be written outside the intended cache location or to an unsafe destination name. This issue is fixed in version 2.7.8.