tests: address two null argument instances

Handle two null argument cases that occur in the unit tests.
One is in library code, the other is in test code.

Detected by running unit tests with undefined behavior sanitizer:
```bash
 # build
mkdir build && cd build
cmake -DBUILD_CLAR=ON -DCMAKE_C_FLAGS="-fsanitize=address \
-fsanitize=undefined -fstack-usage -static-libasan" ..
cmake --build .

 # run with asan
ASAN_OPTIONS="allocator_may_return_null=1" ./libgit2_clar
...
............../libgit2/src/apply.c:316:3: runtime error: null pointer \
passed as argument 1, which is declared to never be null
...................../libgit2/tests/apply/fromfile.c:46:3: runtime \
error: null pointer passed as argument 1, which is declared to never be null
```

1 files changed, 2 insertions, 1 deletions

diff --git a/src/apply.c b/src/apply.c
index 8c7bb6bf3..fd1689879 100644
--- a/src/apply.c
+++ b/src/apply.c
@@ -312,8 +312,9 @@ static int apply_binary(
 			&patch->binary.old_file)) < 0)
 		goto done;
 
+	/* Verify that the resulting file with the reverse patch applied matches the source file */
 	if (source_len != reverse.size ||
-		memcmp(source, reverse.ptr, source_len) != 0) {
+		(source_len && memcmp(source, reverse.ptr, source_len) != 0)) {
 		error = apply_err("binary patch did not apply cleanly");
 		goto done;
 	}