If grep conducts a fixed-strings search (-F) for a pattern that contains
multibyte characters, then it runs indefinitely. When EXECUTE_FCT finds a match
in the middle of a multibyte character, it is supposed to continue after the
multibyte character (search.c:638-639).  However, the beginning of the next
multibyte character is not found, and mb_start remains unchanged
(search.c:228-256). After beg is assigned mb_start - 1, the loop is continue’d
(search.c:640). The loop exit condition never holds (search.c:632) because beg
never exceeds buf + size, resulting in an infinite loop. 

Examples of Correct Fixes: 
1) Raise an error, if is_mb_middle is unsuccessful in finding the beginning of
   the multi-byte and adjusting mb_start. 
2) Go to after the current match. 

Examples of Incorrect Fixes: 
1) Remove continue (Treating the Symptom). 
2) Do not reset beg (Regression because it breaks multibyte character
   handling).