Stackanalysis asserts when analyzing __start_context in libc

[mxz297]

@morehouse Using current master, I encountered an assertion failure from Stackanalysis when analyzing function "__start_context" in libc. I first observed this assertion failure when I was using parseThat to instrument every function in libc and this assertion failure can be reproduced by just invoking stack analysis on function "__start_context".

#0  0x00007ffff5a915f7 in raise () from /lib64/libc.so.6
#1  0x00007ffff5a92ce8 in abort () from /lib64/libc.so.6
#2  0x00007ffff5a8a566 in __assert_fail_base () from /lib64/libc.so.6
#3  0x00007ffff5a8a612 in __assert_fail () from /lib64/libc.so.6
#4  0x00007ffff79dbfc2 in Dyninst::StackAnalysis::summarize (this=this@entry=0x7fffffffd5d0) at /p/paradyn/development/xmeng/dyninstapi/dyninst_development/dyninst/dataflowAPI/src/stackanalysis.C:520
#5  0x00007ffff79dcede in Dyninst::StackAnalysis::analyze (this=0x7fffffffd5d0) at /p/paradyn/development/xmeng/dyninstapi/dyninst_development/dyninst/dataflowAPI/src/stackanalysis.C:100
#6  0x00007ffff79dd57a in Dyninst::StackAnalysis::find (this=this@entry=0x7fffffffd5d0, b=b@entry=0xe57f30, addr=addr@entry=269859698928, loc=...)
    at /p/paradyn/development/xmeng/dyninstapi/dyninst_development/dyninst/dataflowAPI/src/stackanalysis.C:806
#7  0x00007ffff79dd625 in Dyninst::StackAnalysis::findSP (this=this@entry=0x7fffffffd5d0, b=0xe57f30, addr=269859698928) at /p/paradyn/development/xmeng/dyninstapi/dyninst_development/dyninst/dataflowAPI/src/stackanalysis.C:839
#8  0x000000000040111b in main (argc=<optimized out>, argv=0x7fffffffd8b8) at StackAnalysis.cpp:33