trailer: check for memory allocation errors

The "trailer.c" code has been copied mostly verbatim from git.git with
minor adjustments, only. As git.git's `xmalloc` function, which aborts
on memory allocation errors, has been swapped out for `git_malloc`,
which doesn't abort, we may inadvertently access `NULL` pointers.

Add checks to fix this.

src/trailer.c

@@ -267,6 +267,9 @@ static char *extract_trailer_block(const char *message, size_t* len)
 	size_t trailer_len = trailer_end - trailer_start;
 
 	char *buffer = git__malloc(trailer_len + 1);
+	if (buffer == NULL)
+		return NULL;
+
 	memcpy(buffer, message + trailer_start, trailer_len);
 	buffer[trailer_len] = 0;
 
@@ -302,6 +305,8 @@ int git_message_trailers(git_message_trailer_array *trailer_arr, const char *mes
 
 	size_t trailer_len;
 	char *trailer = extract_trailer_block(message, &trailer_len);
+	if (trailer == NULL)
+		return -1;
 
 	for (ptr = trailer;;) {
 		switch (state) {