patch 8.2.4974: ":so" command may read after end of buffer

Problem:    ":so" command may read after end of buffer.
Solution:   Compute length of text properly.

src/scriptfile.c

@@ -1965,15 +1965,16 @@ get_one_sourceline(source_cookie_T *sp)
 		break;
 	    buf = (char_u *)ga.ga_data;
 	    buf[ga.ga_len++] = NUL;
+	    len = ga.ga_len;
 	}
 	else
 	{
 	    buf = (char_u *)ga.ga_data;
 	    if (fgets((char *)buf + ga.ga_len, ga.ga_maxlen - ga.ga_len,
 			sp->fp) == NULL)
 		break;
+	    len = ga.ga_len + (int)STRLEN(buf + ga.ga_len);
 	}
-	len = ga.ga_len + (int)STRLEN(buf + ga.ga_len);
 #ifdef USE_CRNL
 	// Ignore a trailing CTRL-Z, when in Dos mode.	Only recognize the
 	// CTRL-Z by its own, or after a NL.

src/testdir/test_source.vim

@@ -652,6 +652,17 @@ func Test_source_buffer_long_line()
   norm300gr0
   so
   bwipe!
+
+  let lines =<< trim END
+      new
+      norm 10a0000000000ø00000000000
+      norm i0000000000000000000
+      silent! so
+  END
+  call writefile(lines, 'Xtest.vim')
+  source Xtest.vim
+  bwipe!
+  call delete('Xtest.vim')
 endfunc
 
 

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4974,
 /**/
     4973,
 /**/