Fix an infinite loop in c-beginning-of-defun-1

Fix an infinite loop in c-beginning-of-defun-1

This fixes bug#61436.

The loop manifested itself in c-get-fallback-scan-pos as the
position returned wasn't at a {, so the search back in
c-beginning-of-defun-1 with a defun-prompt-regexp found the
same BOL and moved forward to after the match, repeat.

* lisp/progmodes/cc-defs.el (c-beginning-of-defun-1): Accept a
position found by beginning-of-defun (which uses
defun-prompt-regexp) only when the \\s( following it is a {.
Repeat the backward search when this isn't the case.