[gdb/testsuite] Catch condition evaluation errors in gdb_assert

[gdb/testsuite] Catch condition evaluation errors in gdb_assert

When running test-case gdb.base/watchpoint-stops-at-right-insn.exp, we may run
into a tcl error, which can be reproduced reliably using this trigger patch:
...
+       set hw_watch_pc ""
        gdb_assert {$sw_watch_pc == $hw_watch_pc} \
	  "hw watchpoint stops at right instruction"
...
such that we have:
...
ERROR: tcl error sourcing watchpoint-stops-at-right-insn.exp.
ERROR: missing operand at _@_
in expression "0x4004b7 == _@_"
    (parsing expression "0x4004b7 == ")
    invoked from within
"expr $sw_watch_pc == $hw_watch_pc"
    ("uplevel" body line 1)
    invoked from within
"uplevel 1 expr $condition"
    (procedure "gdb_assert" line 6)
    invoked from within
"gdb_assert {$sw_watch_pc == $hw_watch_pc} \
  "hw watchpoint stops at right instruction""
...

A similar problem was fixed in commit 5f0e2eb79e "GDB/testsuite: Fix a
catastrophic step-over-no-symbols.exp failure", by making the assert condition
more robust:
...
-    gdb_assert {$before_addr != $after_addr} "advanced"
+    gdb_assert {{[string is integer -strict $before_addr] \
+                && [string is integer -strict $after_addr] \
+                && $before_addr != $after_addr}} "advanced"
...

Fix this instead in gdb_assert, by catching errors while evaluating the assert
condition.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-09-16  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26624
	* lib/gdb.exp (gdb_assert): Catch errors in condition evaluation.