fix infinite-loop in lsp-bridge-elisp-get-filepath when yaml-mode

[i-aki-y]

The lsp-bridge-elisp-get-filepath appears to enter infinite-loop when yaml-mode.

How to reproduce the problem

1. setup a yaml lsp server (npm install -g yaml-language-server)
2. open a yaml file (test.yaml)
3. add some valid items (a: 1)
4. move the cursor to beginning-of-buffer
5. type any characters, see the example below.

ex.

  <- type here in yaml model
a: 1
b: 2
c: 3

About the fix

In the while loop of the lsp-bridge-elisp-get-filepath function, if file-beg variable has 1 and the substring does not match any existing file paths, the result of (goto-char (1- file-beg)) would not change cursor position (file-beg is still 1) and repeat the same loop infinitely.

I added a check to see whether the file-beg variable reaches the beginning-of-buffer or not.

I could not observe the problem in other language like python-mode.
The yaml seems to be special because a string without quotes (" or ') could be recognized as a valid string. In other languages, strings would be enclosed by quotes "abc" or 'abc', which may avoid the variable file-beg reaches to 1

I would appreciate it if you could review this PR.

[manateelazycat]

Thanks for patch