gdb: bfd_get_symbol_leading_char vs. ""

Some places matching the first char of a string against
bfd_get_symbol_leading_char, which may be zero, didn't check for "".
This could lead to accesses past the end of the string and potential
buffer overruns.  Fix that, and also get rid of a stupid optimisation
in dbxread when looking for "__DYNAMIC" that also might access past
the end of a string.