[glibc] string: Improve memchr random test

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=cdbefe2f940d0ef87c854e13514812785b062988

commit cdbefe2f940d0ef87c854e13514812785b062988
Author: Wilco Dijkstra <[email protected]>
Date:   Thu Jun 18 17:02:59 2026 +0000

    string: Improve memchr random test
    
    Add matches before the start to ensure the memchr test checks they do not
    result in false matches or missed matches.
    
    Reviewed-by: Adhemerval Zanella  <[email protected]>

Diff:
---
 string/test-memchr.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/string/test-memchr.c b/string/test-memchr.c
index df9ce0f22c..42d18a596f 100644
--- a/string/test-memchr.c
+++ b/string/test-memchr.c
@@ -177,6 +177,11 @@ do_random_tests (void)
 	    }
 	}
 
+      /* Add matching chars before the start to improve test coverage.  */
+      i = random () & 31;
+      if (i < align)
+	p[i] = seek_char;
+
       if (pos < len)
 	{
 	  size_t r = random ();