regexp: Silence V595: potential null dereference

The code uses 2-iteration loop antipattern: retval is NULL on first iteration, 
not NULL on second, yet this is still a false positive.

src/nvim/regexp.c

@@ -6928,9 +6928,10 @@ char_u *reg_submatch(int no)
           STRNCPY(retval + len, reg_getline_submatch(lnum),
               submatch_mmatch->endpos[no].col);
         len += submatch_mmatch->endpos[no].col;
-        if (round == 2)
-          retval[len] = NUL;
-        ++len;
+        if (round == 2) {
+          retval[len] = NUL;  // -V595
+        }
+        len++;
       }
 
       if (retval == NULL) {