fix dynamic linker handling of empty path file or error reading path file

previously, the path string was being used despite being invalid. with
this change, empty path file or error reading the path file is treated
as an empty path. this is preferable to falling back to a default
path, so that attacks to prevent reading of the path file could not
result in loading incorrect and possibly dangerous (outdated or
mismatching ABI) libraries from.

the code to strip the final newline has also been removed; now that
newline is accepted as a delimiter, it's harmless to leave it in
place.

src/ldso/dynlink.c

@@ -478,10 +478,9 @@ static struct dso *load_library(const char *name)
 			if (!sys_path) {
 				FILE *f = fopen(ETC_LDSO_PATH, "rbe");
 				if (f) {
-					if (getdelim(&sys_path, (size_t[1]){0}, 0, f) > 0) {
-						size_t l = strlen(sys_path);
-						if (l && sys_path[l-1]=='\n')
-							sys_path[l-1] = 0;
+					if (getdelim(&sys_path, (size_t[1]){0}, 0, f) <= 0) {
+						if (sys_path) free(sys_path);
+						sys_path = "";
 					}
 					fclose(f);
 				}