Fix ELF build ID getter to check the ELF note's namespace string

Fix ELF build ID getter to check the ELF note's namespace string

Otherwise n_type might match a note ID value for a namespace other than
"GNU".

Also add some pedantic bounds checks.

BUG=none
TEST=FileIDTest

[mseaborn]

I noticed this because Roland pointed out I wasn't checking this in some NaCl
code (in https://codereview.chromium.org/14757008/).

[Mark Mentovai]

LGTM

https://breakpad.appspot.com/595002/diff/1/src/common/linux/file_id.cc
File src/common/linux/file_id.cc (right):

https://breakpad.appspot.com/595002/diff/1/src/common/linux/file_id.cc#newcode68
src/common/linux/file_id.cc:68: while (reinterpret_cast<const
void*>(note_header) < section_end) {
I wish this whole section wasn’t so casty. Oh well.

https://breakpad.appspot.com/595002/diff/1/src/common/linux/file_id.cc#newcode75
src/common/linux/file_id.cc:75: if (reinterpret_cast<const void*>(note_end) >
section_end)
I don’t know if you need to check both name_field and note_end against
section_end. You can maybe get away with dumping the check on line 70, unless
you’re worried about overflow.

https://breakpad.appspot.com/595002/diff/1/src/common/linux/file_id.cc#newcode80
src/common/linux/file_id.cc:80: break;
{} this because the condition’s already taken up so many lines.