patch 9.1.0426: too many strlen() calls in search.c

Problem:  too many strlen() calls in search.c
Solution: refactor code and remove more strlen() calls,
          use explicit variable to remember strlen
          (John Marriott)

closes: #14796

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/structs.h

@@ -4795,6 +4795,7 @@ typedef struct soffset
 typedef struct spat
 {
     char_u	    *pat;	// the pattern (in allocated memory) or NULL
+    size_t	    patlen;	// the length of the patten (0 is pat is NULL)
     int		    magic;	// magicness of the pattern
     int		    no_scs;	// no smartcase for this pattern
     soffset_T	    off;