Handle OP_STRING in dump_subexp_body_standard

Handle OP_STRING in dump_subexp_body_standard

For some reason, OP_STRING is not handled in dump_subexp_body_standard.
This makes the output of "set debug expression 1" very bad when a string
is involved. Example:

(gdb) set debug expression 1
(gdb) print "hello"
... (random garbage, possibly segfault)

This commit handles OP_STRING and skips the appropriate number of exp
elements. The line corresponding to the string now looks like:

	    0  OP_STRING             Language-specific string type: 0

gdb/ChangeLog:

2014-07-15  Simon Marchi  <simon.marchi@ericsson.com>

	* expprint.c (dump_subexp_body_standard): Handle OP_STRING.