gdb: avoid nullptr access in dbxread.c from read_dbx_symtab

This fixes a GDB crash reported in bug pr/28900, related to reading in
some stabs debug information.

In this commit my goal is to stop GDB crashing.  I am not trying to
ensure that GDB makes the best possible use of the available stabs
debug information.  At this point I consider stabs a legacy debug
format, with only limited support in GDB.

So, the problem appears to be that, when reading in the stabs data, we
need to find a N_SO entry, this is the entry that defines the start of
a compilation unit (or at least the location of a corresponding source
file).

It is while handling an N_SO that GDB creates a psymtab to hold the
incoming debug information (symbols, etc).

The problem we hit in the bug is that we encounter some symbol
information (an N_PC entry) outside of an N_SO entry - that is we find
some symbol information that is not associated with any source file.

We already have some protection for this case, look (in
read_dbx_symtab) at the handling of N_PC entries of type 'F' and 'f',
if we have no psymtab (the pst variable is nullptr) then we issue a
complaint.  However, for whatever reason, in both 'f' and 'F'
handling, there is one place where we assume that the pst
variable (the psymtab) is not nullptr.  This is a mistake.

In this commit, I guard these two locations (in 'f' and 'F' handling)
so we no longer assume pst is not nullptr.

While I was at it, I audited all the other uses of pst in
read_dbx_symtab, and in every potentially dangerous case I added a
nullptr check, and issue a suitable complaint if pst is found to be
nullptr.

It might well be true that we could/should do something smarter if we
see a debug symbol outside of an N_SO entry, and if anyone wanted to
do that work, they're welcome too.  But this commit is just about
preventing the nullptr access, and the subsequent GDB crash.

I don't have any tests for this change, I have no idea how to generate
weird stabs data for testing.  The original binary from the bug report
now loads just fine without GDB crashing.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28900

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