patch 8.2.1058: multiline conceal causes display errors

Problem:    Multiline conceal causes display errors.
Solution:   Do not allow conceal cross over EOL. (closes #6326, closes #4854,
            closes #6302)
diff --git a/src/drawline.c b/src/drawline.c
index fad0645..fa25965 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -1334,6 +1334,11 @@
 				      &screen_search_hl, &has_match_conc,
 				      &match_conc, did_line_attr, lcs_eol_one);
 		ptr = line + v;  // "line" may have been changed
+
+		// Do not allow a conceal over EOL otherwise EOL will be missed
+		// and bad things happen.
+		if (*ptr == NUL)
+		    has_match_conc = 0;
 	    }
 #endif