symtab.c change breaks assembly lang debugging

[Michael Snyder]

Jim, 

I presume the change below was meant as an optimization, but it's
based on a false assumption -- that you can't have line numbers
if you don't have block symbols.  That premise is untrue if you
are debugging assembly language source, where the assembler emits
line syms but not block syms.  This change breaks the test case
for assembly source debugging, gdb.asm/asm-source.exp.

You would not have noticed, because that test only works on a few
targets right now (d10v being one of them).

	Patch from Peter Schauer:

	* symtab.c (find_pc_sect_line): If we can't find the function
	containing PC, we certainly won't have line number information for
	that location, so return zero immediately.

Michael