libsysprof-analyze: add CString helper

This checks, up to the end of the frame, that we have a valid \0 for the
CString before passing it back to the caller. Otherwise, NULL is returned
for a corrupt/invalid CString within the frame.

src/libsysprof-analyze/sysprof-document-frame-private.h

@@ -80,4 +80,19 @@ SysprofDocumentFrame *_sysprof_document_frame_new (GMappedFile               *ma
    (SYSPROF_DOCUMENT_FRAME_NEEDS_SWAP(obj) ? GINT64_TO_BE(val) : (val))
 #endif
 
+static inline const char *
+SYSPROF_DOCUMENT_FRAME_CSTRING (SysprofDocumentFrame *self,
+                                const char           *str)
+{
+  const char *endptr = (const char *)self->frame + self->frame_len;
+
+  for (const char *c = str; c < endptr; c++)
+    {
+      if (*c == 0)
+        return str;
+    }
+
+  return NULL;
+}
+
 G_END_DECLS