buffer: return errors for `git_buf_init` and `git_buf_attach`

Both the `git_buf_init` and `git_buf_attach` functions may call
`git_buf_grow` in case they were given an allocation length as
parameter. As such, it is possible for these functions to fail when we
run out of memory. While it won't probably be used anytime soon, it does
indeed make sense to also record this fact by returning an error code
from both functions. As they belong to the internal API only, this
change does not break our interface.
diff --git a/src/buffer.c b/src/buffer.c
index 40bed5c..6dfcbfb 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -22,14 +22,15 @@
 		return -1;
 
 
-void git_buf_init(git_buf *buf, size_t initial_size)
+int git_buf_init(git_buf *buf, size_t initial_size)
 {
 	buf->asize = 0;
 	buf->size = 0;
 	buf->ptr = git_buf__initbuf;
 
-	if (initial_size)
-		git_buf_grow(buf, initial_size);
+	ENSURE_SIZE(buf, initial_size);
+
+	return 0;
 }
 
 int git_buf_try_grow(
@@ -577,7 +578,7 @@
 	return data;
 }
 
-void git_buf_attach(git_buf *buf, char *ptr, size_t asize)
+int git_buf_attach(git_buf *buf, char *ptr, size_t asize)
 {
 	git_buf_free(buf);
 
@@ -588,9 +589,10 @@
 			buf->asize = (asize < buf->size) ? buf->size + 1 : asize;
 		else /* pass 0 to fall back on strlen + 1 */
 			buf->asize = buf->size + 1;
-	} else {
-		git_buf_grow(buf, asize);
 	}
+
+	ENSURE_SIZE(buf, asize);
+	return 0;
 }
 
 void git_buf_attach_notowned(git_buf *buf, const char *ptr, size_t size)
diff --git a/src/buffer.h b/src/buffer.h
index a76b2d7..b0aece4 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -34,7 +34,7 @@
  * For the cases where GIT_BUF_INIT cannot be used to do static
  * initialization.
  */
-extern void git_buf_init(git_buf *buf, size_t initial_size);
+extern int git_buf_init(git_buf *buf, size_t initial_size);
 
 /**
  * Resize the buffer allocation to make more space.
@@ -73,7 +73,7 @@
 
 extern void git_buf_swap(git_buf *buf_a, git_buf *buf_b);
 extern char *git_buf_detach(git_buf *buf);
-extern void git_buf_attach(git_buf *buf, char *ptr, size_t asize);
+extern int git_buf_attach(git_buf *buf, char *ptr, size_t asize);
 
 /* Populates a `git_buf` where the contents are not "owned" by the
  * buffer, and calls to `git_buf_free` will not free the given buf.