global init: check error message buffer allocation

Ensure that we can allocate the error message buffer.  In keeping with
our typical policiess, we allow (small) memory leaks in the case where
we're out of memory.

src/global.c

@@ -203,7 +203,8 @@ git_global_st *git__global_state(void)
 	if (!ptr)
 		return NULL;
 
-	git_buf_init(&ptr->error_buf, 0);
+	if (git_buf_init(&ptr->error_buf, 0) < 0)
+		return NULL;
 
 	FlsSetValue(_fls_index, ptr);
 	return ptr;
@@ -289,7 +290,9 @@ git_global_st *git__global_state(void)
 	if (!ptr)
 		return NULL;
 
-	git_buf_init(&ptr->error_buf, 0);
+	if (git_buf_init(&ptr->error_buf, 0) < 0)
+		return NULL;
+
 	pthread_setspecific(_tls_key, ptr);
 	return ptr;
 }