buffer: use `git_buf_init` with length

The `git_buf_init` function has an optional length parameter, which will
cause the buffer to be initialized and allocated in one step. This can
be used instead of static initialization with `GIT_BUF_INIT` followed by
a `git_buf_grow`. This patch does so for two functions where it is
applicable.

src/config_file.c

@@ -1267,7 +1267,7 @@ static const char *escaped = "\n\t\b\"\\";
 /* Escape the values to write them to the file */
 static char *escape_value(const char *ptr)
 {
-	git_buf buf = GIT_BUF_INIT;
+	git_buf buf;
 	size_t len;
 	const char *esc;
 
@@ -1277,7 +1277,8 @@ static char *escape_value(const char *ptr)
 	if (!len)
 		return git__calloc(1, sizeof(char));
 
-	git_buf_grow(&buf, len);
+	if (git_buf_init(&buf, len) < 0)
+		return NULL;
 
 	while (*ptr != '\0') {
 		if ((esc = strchr(escaped, *ptr)) != NULL) {

src/pack.c

@@ -312,7 +312,7 @@ static int pack_index_open(struct git_pack_file *p)
 {
 	int error = 0;
 	size_t name_len;
-	git_buf idx_name = GIT_BUF_INIT;
+	git_buf idx_name;
 
 	if (p->index_version > -1)
 		return 0;
@@ -320,10 +320,13 @@ static int pack_index_open(struct git_pack_file *p)
 	name_len = strlen(p->pack_name);
 	assert(name_len > strlen(".pack")); /* checked by git_pack_file alloc */
 
-	git_buf_grow(&idx_name, name_len);
+	if (git_buf_init(&idx_name, name_len) < 0)
+		return -1;
+
 	git_buf_put(&idx_name, p->pack_name, name_len - strlen(".pack"));
 	git_buf_puts(&idx_name, ".idx");
 	if (git_buf_oom(&idx_name)) {
+		git_buf_free(&idx_name);
 		return -1;
 	}