Fix an error reporting bug in "Checksum missing or mismatched ..."

The sense of the test was accidentally inverted in my change to #4733.
The message should be shown if any of the files exist but have an incorrect checksum.
We also now correctly handle the case where there are no package source files.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

depends/Makefile

@@ -138,7 +138,7 @@ endef
 define check_or_remove_sources
   mkdir -p $($(package)_source_dir); cd $($(package)_source_dir); \
   $(build_SHA256SUM) -c $($(package)_fetched) >/dev/null 2>/dev/null || \
-    ( ( echo $($(package)_all_sources) | xargs -n 1 test -f ) || echo "Checksum missing or mismatched for $(package) source. Forcing re-download."; \
+    ( test -z "$($(package)_all_sources)" || ( echo $($(package)_all_sources) | xargs -n 1 test ! -f ) || echo "Checksum missing or mismatched for $(package) source. Forcing re-download."; \
       rm -f $($(package)_all_sources) $($(1)_fetched))
 endef