Correct beginning of inline code position

markdown-regex-code starts with "\(?:\`\|[^\]\)\(\(`+\)...". If cursor is
beginning of buffer, "(match-beginning 0)" is on backtick. While cursor is not
beginning of buffer, "(match-beginning 0)" is not on backtick and previous point
of backtick.

---
foo:bar
---
`x`

In above example, (match-beginning 0) is point of after '---'. That point is
a code block and '(markdown-code-block-at-pos pos)' returns non-nil,
this causes infinite loop of markdown-match-inline-generic.
'(match-beginning 1)' of markdown-regex-code always on backtick. We should
use '(match-beginning 1)' instead of '(match-beginning 0') in such case.

markdown-mode.el

@@ -2805,7 +2805,7 @@ Return nil otherwise."
 (defun markdown-match-inline-generic (regex last)
   "Match inline REGEX from the point to LAST."
   (when (re-search-forward regex last t)
-    (let ((bounds (markdown-code-block-at-pos (match-beginning 0))))
+    (let ((bounds (markdown-code-block-at-pos (match-beginning 1))))
       (if (null bounds)
           ;; Not in a code block: keep match data and return t when in bounds
           (<= (match-end 0) last)