Robustify gdb.base/testenv.exp:find_env

Robustify gdb.base/testenv.exp:find_env

If printing 'envp[$i]' in find_env fails, the testcase goes into an
infinite loop:

 Running /home/pedro/rocm/gdb/build/gdb/testsuite/../../../src/gdb/testsuite/gdb.base/testenv.exp ...
 FAIL: gdb.base/testenv.exp: test_inherit_env_var: test1: printf "var1: %s\n", envp[0] ? envp[0] : ""
 FAIL: gdb.base/testenv.exp: test_inherit_env_var: test1: printf "var1: %s\n", envp[1] ? envp[1] : ""
 FAIL: gdb.base/testenv.exp: test_inherit_env_var: test1: printf "var1: %s\n", envp[2] ? envp[2] : ""
 FAIL: gdb.base/testenv.exp: test_inherit_env_var: test1: printf "var1: %s\n", envp[3] ? envp[3] : ""
 ...
 FAIL: gdb.base/testenv.exp: test_inherit_env_var: test1: printf "var1: %s\n", envp[2519] ? envp[2519] : ""
 FAIL: gdb.base/testenv.exp: test_inherit_env_var: test1: printf "var1: %s\n", envp[2520] ? envp[2520] : ""
 FAIL: gdb.base/testenv.exp: test_inherit_env_var: test1: printf "var1: %s\n", envp[2521] ? envp[2521] : ""
 ..
 FAIL: gdb.base/testenv.exp: test_inherit_env_var: test1: printf "var1: %s\n", envp[5128] ? envp[5128] : ""
 FAIL: gdb.base/testenv.exp: test_inherit_env_var: test1: printf "var1: %s\n", envp[5129] ? envp[5129] : ""
 FAIL: gdb.base/testenv.exp: test_inherit_env_var: test1: printf "var1: %s\n", envp[5130] ? envp[5130] : ""
 ...

Fix that by bailing out of the loop on failure.

gdb/testsuite/ChangeLog:

	* gdb.base/testenv.exp (find_env): Bail out if printing 'envp[$i]'
	fails.