dereference near-null pointer in formUpdateBuffer

[kcwu]

input

00000000: 3c74 6162 6c65 3e30 3c6e 6f62 722f 3c3e  <table>0<nobr/<>
00000010: 303c 786d 703e 3c69 6e74 6572 6e61 6c3e  0<xmp><internal>
00000020: 3c73 656c 6563 7420 6d75 6c74 6970 6c65  <select multiple
00000030: 3e3c 6f70 7469 6f6e 3e                   ><option>

gdb --args ./w3m -T text/html -dump file

Program received signal SIGSEGV, Segmentation fault.
0x000000000044f741 in formUpdateBuffer (a=0x7e1000, buf=0x7d4e00, form=0x7e0f80) at form.c:445
445             if (spos >= buf->currentLine->len || spos < 0)
(gdb) p buf
$1 = (Buffer *) 0x7d4e00
(gdb) p buf->currentLine
$2 = (Line *) 0x0
(gdb) bt
#0  0x000000000044f741 in formUpdateBuffer (a=0x7e1000, buf=0x7d4e00, form=0x7e0f80) at form.c:445
#1  0x000000000044f04c in formResetBuffer (buf=0x7d4e00, formitem=0x7db500) at form.c:272
#2  0x000000000042c9cc in loadHTMLBuffer (f=0x7fffffffca80, newBuf=0x7d4e00) at file.c:6781
#3  0x0000000000416ae0 in loadSomething (f=0x7fffffffca80, loadproc=0x42c85e <loadHTMLBuffer>, defaultbuf=0x7d4e00) at file.c:224
#4  0x000000000041c952 in loadGeneralFile (path=0x7c4b00 "min/32", current=0x0, referer=0xffffffffffffffff <error: Cannot access memory at address 0xffffffffffffffff>, flag=0, request=0x0) at file.c:2241
#5  0x0000000000407171 in main (argc=5, argv=0x7fffffffcda8, envp=0x7fffffffcdd8) at main.c:1020

found by afl-fuzz

[tats]

Fixed, thank you.