Use after free in qmake

qmake has a use after free error that can be exposed with address sanitizer. While the qt build system has an option to use address sanitizer this does not automatically build qmake with it, which is probably the reason this bug wasn't detected before.

To reproduce:

• unpack qt source, cd to it
• Add address sanitizer flags:
  echo "QMAKE_CFLAGS += -fsanitize=address -g" >> mkspecs/common/gcc-base.conf
  echo "QMAKE_CXXFLAGS += -fsanitize=address -g" >> mkspecs/common/gcc-base.conf
  echo "QMAKE_LFLAGS += -fsanitize=address -g" >> mkspecs/common/gcc-base.conf
• Run configure and thus build qmake:
  ./configure -opensource -confirm-license

The error will show up as soon as qmake was built and started within the QT source dir. I've attached a full stack trace from address sanitizer (for a more detailed trace use ASAN_OPTIONS="fast_unwind_on_malloc=0").

I have tried to analyze the bug, but the code is pretty complicated and I'm not familiar with it.