patch 9.0.0407: matchstr() does match column offset

Problem:    matchstr() does match column offset. (Yasuhiro Matsumoto)
Solution:   Accept line number zero. (closes #10938)

src/regexp_nfa.c

@@ -6778,7 +6778,7 @@ nfa_regmatch(
 			linenr_T    lnum = rex.reg_firstlnum + rex.lnum;
 			long_u	    vcol = 0;
 
-			if (lnum > 0
+			if (lnum >= 0
 				   && lnum <= wp->w_buffer->b_ml.ml_line_count)
 			    vcol = (long_u)win_linetabsize(wp, lnum,
 								rex.line, col);