Fix crash when all glyphs of a given depth are freed, but not all glyphsets

This is how the crash can be triggered with only two clients on the system:
Client A: (already running)
Client B: Connect
Client B: CreateGlyphSet(depthN)
Client A: Disconnect
Server: free globalGlyphs(depthN)
Client B: AddGlyphs(depthN)
Server: SEGV

This crash was introduced with the FindGlyphsByHash function
in 516b96387b. Before that revision,
ResizeGlyphSet was always called before FindGlyphRef, which would
re-create globalGlyphs(depthN) if necessary.

X.Org Bug 20718 <http://bugs.freedesktop.org/show_bug.cgi?id=20718>

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Peter Harris <pharris@opentext.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

render/glyph.c

@@ -217,6 +217,9 @@ FindGlyphByHash (unsigned char sha1[20], int format)
     GlyphRefPtr gr;
     CARD32 signature = *(CARD32 *) sha1;
 
+    if (!globalGlyphs[format].hashSet)
+	return NULL;
+
     gr = FindGlyphRef (&globalGlyphs[format],
 		       signature, TRUE, sha1);