revparse: detect out-of-memory cases when parsing curly brace contents

When extracting curly braces (e.g. the "upstream" part in
"HEAD@{upstream}"), we put the curly braces' contents into a `git_buf`
structure, but don't check the return value of `git_buf_putc`. So when
we run out-of-memory, we'll use a partially filled buffer without
noticing.

Let's fix this issue by checking `git_buf_putc`'s return value.

src/revparse.c

@@ -537,7 +537,8 @@ static int extract_curly_braces_content(git_buf *buf, const char *spec, size_t *
 		if (spec[*pos] == '\0')
 			return GIT_EINVALIDSPEC;
 
-		git_buf_putc(buf, spec[(*pos)++]);
+		if (git_buf_putc(buf, spec[(*pos)++]) < 0)
+			return -1;
 	}
 
 	(*pos)++;