Detect whatever a built-in has been corrupted by means of cmsWriteRawTag

Thanks to @prrace to detect such subtle bug.
cmsWriteRawTag  could cause a segfault when used to corrupt a tag on a built-in profile-

1 files changed, 7 insertions, 0 deletions

diff --git a/src/cmsio0.c b/src/cmsio0.c
index 0143efb..e369e0d 100644
--- a/src/cmsio0.c
+++ b/src/cmsio0.c
@@ -1558,6 +1558,13 @@ void* CMSEXPORT cmsReadTag(cmsHPROFILE hProfile, cmsTagSignature sig)
     if (TagSize < 8) goto Error;
 
     io = Icc ->IOhandler;
+
+    if (io == NULL) { // This is a built-in profile that has been manipulated, abort early
+
+        cmsSignalError(Icc->ContextID, cmsERROR_CORRUPTION_DETECTED, "Corrupted built-in profile.");
+        goto Error;
+    }
+
     // Seek to its location
     if (!io -> Seek(io, Offset))
         goto Error;