Fix the stack buffer overflow issue

strlen() could returns 0. Without a conditional check for len,
accessing S_ pointer with len - 1 may causes a stack buffer overflow.

AddressSanitizer reports this like:
==1219243==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffdce043c1f at pc 0x000000403547 bp 0x7ffdce0
43b30 sp 0x7ffdce043b28
READ of size 1 at 0x7ffdce043c1f thread T0
    #0 0x403546 in main ../bin/fribidi-main.c:393
    #1 0x7f226804e58f in __libc_start_call_main (/lib64/libc.so.6+0x2d58f)
    #2 0x7f226804e648 in __libc_start_main_impl (/lib64/libc.so.6+0x2d648)
    #3 0x4036f4 in _start (/tmp/fribidi/build/bin/fribidi+0x4036f4)

Address 0x7ffdce043c1f is located in stack of thread T0 at offset 63 in frame
    #0 0x4022bf in main ../bin/fribidi-main.c:193

  This frame has 5 object(s):
    [32, 36) 'option_index' (line 233)
    [48, 52) 'base' (line 386)
    [64, 65064) 'S_' (line 375) <== Memory access at offset 63 underflows this variable
    [65328, 130328) 'outstring' (line 385)
    [130592, 390592) 'logical' (line 384)

This fixes #181

bin/fribidi-main.c

@@ -390,7 +390,7 @@ FRIBIDI_END_IGNORE_DEPRECATIONS
 	    S_[sizeof (S_) - 1] = 0;
 	    len = strlen (S_);
 	    /* chop */
-	    if (S_[len - 1] == '\n')
+	    if (len > 0 && S_[len - 1] == '\n')
 	      {
 		len--;
 		S_[len] = '\0';