RFR (S) 8144491 ElfSymbolTable::lookup returns bad value

[Ioi Lam]

Please review a small fix:

http://cr.openjdk.java.net/~iklam/8144491-ElfSymbolTable-lookup/

Bug: ElfSymbolTable::lookup returns bad value when the lookup has failed

     https://bugs.openjdk.java.net/browse/JDK-8144491

Summary of fix:

     I found this when trying to make disassembler.cpp print out more 
symbolic
     information for PrintInterpreter.

     The fix is straight-forward -- if the lookup fails, 
ElfSymbolTable::lookup
     should return false.

     Impact:
     The only caller to ElfSymbolTable::lookup is ElfFile::decode(), so 
nothing
     else would depend on the old (bad) behavior of ElfSymbolTable::lookup.

     I also took the chance to refactor lookup is ElfFile::decode() to 
split out
     two blocks of identical code into a new function, 
ElfSymbolTable::compare.

Tests:

     JPRT
     RBT (hotspot/test/:hotspot_all)  <-- what other tests should I run?

Thanks
- Ioi