fix for #76200. crash when viewing a message with an <img> tag but no src.

r/sr=valeski,mscott.  a=blizzard

mailnews/base/util/nsMsgMailNewsUrl.cpp

@@ -482,7 +482,22 @@ NS_IMETHODIMP nsMsgMailNewsUrl::Equals(nsIURI *other, PRBool *_retval)
 
 NS_IMETHODIMP nsMsgMailNewsUrl::SchemeIs(const char *aScheme, PRBool *_retval)
 {
+  nsXPIDLCString scheme;
+  nsresult rv = m_baseURL->GetScheme(getter_Copies(scheme));
+  NS_ENSURE_SUCCESS(rv,rv);
+
+  // fix #76200 crash on email with <img> with no src.
+  //
+  // make sure we have a scheme before calling SchemeIs()
+  // we have to do this because url parsing can result in a null mScheme
+  // this extra string copy should be removed when #73845 is fixed.
+  if (scheme.get()) {
     return m_baseURL->SchemeIs(aScheme, _retval);
+  }
+  else {
+    *_retval = PR_FALSE;
+    return NS_OK;
+  }
 }
 
 NS_IMETHODIMP nsMsgMailNewsUrl::Clone(nsIURI **_retval)