patch 8.2.3662: illegal memory access if malloc() fails

Problem:    Illegal memory access if malloc() fails.
Solution:   Check 'foldmethod' is not empty. (closes #9207)

Author: brammool

src/fold.c

@@ -307,7 +307,7 @@ foldedCount(win_T *win, linenr_T lnum, foldinfo_T *infop)
     int
 foldmethodIsManual(win_T *wp)
 {
-    return (wp->w_p_fdm[3] == 'u');
+    return (wp->w_p_fdm[0] != NUL && wp->w_p_fdm[3] == 'u');
 }
 
 // foldmethodIsIndent() {{{2
@@ -327,7 +327,7 @@ foldmethodIsIndent(win_T *wp)
     int
 foldmethodIsExpr(win_T *wp)
 {
-    return (wp->w_p_fdm[1] == 'x');
+    return (wp->w_p_fdm[0] != NUL && wp->w_p_fdm[1] == 'x');
 }
 
 // foldmethodIsMarker() {{{2
@@ -337,7 +337,7 @@ foldmethodIsExpr(win_T *wp)
     int
 foldmethodIsMarker(win_T *wp)
 {
-    return (wp->w_p_fdm[2] == 'r');
+    return (wp->w_p_fdm[0] != NUL && wp->w_p_fdm[2] == 'r');
 }
 
 // foldmethodIsSyntax() {{{2

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3662,
 /**/
     3661,
 /**/