add an extra inbounds in nextind to prevent a gc frame (#35827)

* add an extra inbounds in nextind to prevent a gc frame

* another inbounds

Author: KristofferC

base/strings/string.jl

@@ -144,8 +144,8 @@ end
     @inbounds l = codeunit(s, i)
     (l < 0x80) | (0xf8 ≤ l) && return i+1
     if l < 0xc0
-        i′ = thisind(s, i)
-        return i′ < i ? nextind(s, i′) : i+1
+        i′ = @inbounds thisind(s, i)
+        return i′ < i ? @inbounds(nextind(s, i′)) : i+1
     end
     # first continuation byte
     (i += 1) > n && return i