spirv-as hangs in spvReadEnvironmentFromText

[svenvh]

Adding some comments to the top of a spvasm file without providing a --target-env flag causes spirv-as to hang inside spvReadEnvironmentFromText; presumably caused by #5893 .

To reproduce, apply the following patch to add a test case:

diff --git a/test/target_env_test.cpp b/test/target_env_test.cpp
index faffa8a1..eb06a691 100644
--- a/test/target_env_test.cpp
+++ b/test/target_env_test.cpp
@@ -221,6 +221,7 @@ INSTANTIATE_TEST_SUITE_P(
         {"     \t ; Version: 1.1", true, SPV_ENV_UNIVERSAL_1_1},
         // Previous lines
         {"; SPIR-V\n; Version: 1.1", true, SPV_ENV_UNIVERSAL_1_1},
+        {"; -\n; SPIR-V\n; Version: 1.1", true, SPV_ENV_UNIVERSAL_1_1},
 
         // After a non-header line
         {"OpCapability Shader\n; Version: 1.1", false, kSentinelEnv}}));

Then build and run the test/test_spirv_unit_tests test suite. The TargetTextParsing/TargetParseEnvInDisassemblyTest test will hang.