mkfontscale: Avoid a buffer underrun

https://bugs.freedesktop.org/show_bug.cgi?id=96905

=================================================================
==3068==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60300000de6f at pc 0x000109df79d1 bp 0x7fff55e37d90 sp 0x7fff55e37550
READ of size 1 at 0x60300000de6f thread T0
    #0 0x109df79d0 in wrap_strcmp (libclang_rt.asan_osx_dynamic.dylib+0xe9d0)
    #1 0x109dc8ef7 in fontFileOpen (mkfontscale+0x100001ef7)
    #2 0x109dc8e0b in bitmapIdentify (mkfontscale+0x100001e0b)
    #3 0x109dcc874 in doDirectory (mkfontscale+0x100005874)
    #4 0x109dcbc78 in main (mkfontscale+0x100004c78)
    #5 0x7fff9083f5ac in start (libdyld.dylib+0x35ac)
    #6 0x3  (<unknown module>)

0x60300000de6f is located 1 bytes to the left of 20-byte region [0x60300000de70,0x60300000de84)
allocated by thread T0 here:
    #0 0x109e34570 in wrap_malloc (libclang_rt.asan_osx_dynamic.dylib+0x4b570)
    #1 0x109dca92a in dsprintf (mkfontscale+0x10000392a)
    #2 0x109dcc788 in doDirectory (mkfontscale+0x100005788)
    #3 0x109dcbc78 in main (mkfontscale+0x100004c78)
    #4 0x7fff9083f5ac in start (libdyld.dylib+0x35ac)
    #5 0x3  (<unknown module>)

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/ident.c b/ident.c
index 22d3648..f7fe9e1 100644
--- a/ident.c
+++ b/ident.c
@@ -83,7 +83,7 @@ static inline void *
 fontFileOpen(fontFile *ff, const char *filename) {
     int n = strlen(filename);
 
-    if (strcmp(filename + n - 4, ".bz2") == 0) {
+    if (n > 4 && strcmp(filename + n - 4, ".bz2") == 0) {
 	ff->type = bz2FontFile;
 	ff->f.bz2 = BZ2_bzopen(filename, "rb");
 	ff->pos = 0;