msxml3: getPrefix() should check all pushed contexts as well.

[wine] / dlls / msxml3 / msxml3.manifest

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
  <assemblyIdentity type="win32" name="Microsoft-Windows-MSXML30" version="6.0.6000.16386" processorArchitecture="" publicKeyToken="31bf3856ad364e35"/>
  <file name="msxml3.dll"/>
</assembly>