transport: prevent git_remote_download() from segfaulting when being passed a…

transport: prevent git_remote_download() from segfaulting when being passed a lightweight remote built with git_remote_new()

src/refspec.c

@@ -57,7 +57,7 @@ const char *git_refspec_dst(const git_refspec *refspec)
 
 int git_refspec_src_match(const git_refspec *refspec, const char *refname)
 {
-	return refspec == NULL ? GIT_ENOMATCH : git__fnmatch(refspec->src, refname, 0);
+	return (refspec == NULL || refspec->src == NULL) ? GIT_ENOMATCH : git__fnmatch(refspec->src, refname, 0);
 }
 
 int git_refspec_transform(char *out, size_t outlen, const git_refspec *spec, const char *name)