Stack overflow in process_raw_blocks

## Problem Description

Note: I am aware that this project is unmaintained. However, I am still opening this issue to follow [CVE's guidelines for EOL software](https://www.cve.org/Resources/General/End-of-Life-EOL-Assignment-Process.pdf).

There is a stack overflow (aka: infinite recursion) in the `process_raw_blocks` function when the parser handles a specially crafted Markdown file. This could be used to perform a denial-of-service attack.

Here is a minimized proof-of-concept Markdown file that triggers the bug: [min_stack_overflow.md](https://github.com/jgm/peg-markdown/files/12413332/min_stack_overflow.md). The output is as follows:

```
#210 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#211 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#212 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#213 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#214 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#215 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#216 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#217 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#218 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#219 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#220 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#221 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#222 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#223 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#224 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#225 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#226 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#227 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#228 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#229 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#230 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#231 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#232 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#233 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#234 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#235 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#236 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#237 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#238 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#239 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#240 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#241 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#242 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#243 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#244 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#245 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#246 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#247 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33
#248 0x5dcd55 in process_raw_blocks /home/sanic/peg-markdown/markdown_lib.c:137:33

SUMMARY: AddressSanitizer: stack-overflow /home/sanic/peg-markdown/markdown_lib.c:113 in process_raw_blocks
==2808508==ABORTING
```

## Reproduction Steps
1. Compile the project using ASAN (Address Sanitizer). For example, `CC=afl-clang-fast AFL_USE_ASAN=1 make`.
2. Run `./markdown -x min_stack_overflow.md` (use the proof-of-concept file attached to this report).
3. Observe the stack overflow in the output.