Fix global buffer overflow in IDLC

This fixes a global buffer overflow (underflow?) reported by ASAN as
follows:

ERROR: AddressSanitizer: global-buffer-overflow on address \
  0x000100144e3f at pc 0x000100084424 bp 0x00016fdfd380 sp 0x00016fdfd378
READ of size 1 at 0x000100144e3f thread T0
    #0 0x100084420 in put_a_line main.c:975
    #1 0x100082974 in putout main.c:895
    eclipse-cyclonedds#2 0x100080ab4 in mcpp_main main.c:753
    eclipse-cyclonedds#3 0x10007d3fc in mcpp_lib_main main.c:430
    eclipse-cyclonedds#4 0x10000a230 in idlc_parse idlc.c:375
    eclipse-cyclonedds#5 0x100006f1c in main idlc.c:772
    eclipse-cyclonedds#6 0x181d51054  (<unknown module>)

0x000100144e3f is located 1 bytes before global variable 'output' \
  defined in 'src/tools/idlpp/src/main.c' (0x100144e40) of size 262144
0x000100144e3f is located 23 bytes after global variable 'src_col' \
  defined in 'src/tools/idlpp/src/main.c' (0x100144e20) of size 8

Triggered by the following input ("hexdump -C" output):
00000000  23 fc ff 0a 23 64 65 66  69 6e 65 20 69 0a 23 64  |#...#define i.#d|
00000010  65 66 69 6e 65 20 4a 00  0a 20 4a 00 0a 23 64 69  |efine J.. J..#di|
00000020  44 66 7f f8 ff ff 66 00  32 44 0a 23 64 69 69 6e  |Df....f.2D.#diin|
00000030  65 20 4a 32 32 32 44 12  64 27 ff 7f ff 23 3e 69  |e J222D.d'...#>i|
00000040  0a                                                |.|
00000041
(I tried to reduce it further, but gave up.)

The problem is caused by scanning for the last non-whitespace character
just before outputting a line without accounting for the case where the
line consists solely of whitespace.

Credits for finding the bug:
- Carlos Andres Ramirez (https://carlos.engineer)
- Goktug Serez (https://github.com/g0ku704)
- Xin Huang (https://github.com/xinhuang)

Signed-off-by: Erik Boasson <eb@ilities.com>

src/tools/idlpp/src/main.c

@@ -965,18 +965,21 @@ static void put_a_line(
  */
 {
     size_t  len;
-    char *  out_p;
-    char *  tp;
 
     if (no_output)
         return;
     len = strlen( out);
-    tp = out_p = out + len - 2;             /* Just before '\n'     */
-    while (char_type[ *out_p & UCHARMAX] & SPA)
-        out_p--;                    /* Remove trailing white spaces */
-    if (out_p < tp) {
-        *++out_p = '\n';
-        *++out_p = EOS;
+    if (len > 2)
+    {
+        char *  out_p;
+        char *  tp;
+        tp = out_p = out + len - 2;         /* Just before '\n'     */
+        while (out_p > out && char_type[ *out_p & UCHARMAX] & SPA)
+            out_p--;                /* Remove trailing white spaces */
+        if (out_p < tp && !(char_type[ *out_p & UCHARMAX] & SPA)) {
+            *++out_p = '\n';
+            *++out_p = EOS;
+        }
     }
     if (mcpp_fputs( out, MCPP_OUT) == EOF)
         cfatal( "File write error", NULL, 0L, NULL);        /* _F_  */