preprocessor: guard against column underflow in DoPreprocessing

When DoPreprocessing::operator() encounters a new line, it attempts to
emit leading whitespace via:

    outputBuffer += std::string(ppToken.loc.column - 1, ' ');

If ppToken.loc.column is 0 (which can occur after a backslash-newline
continuation at the end of input, e.g. 'GL_EXT_shader_image_load_formatted\\n'
with GLSL version 450 and EShMsgDefault), the subtraction underflows to
SIZE_MAX, causing std::string(SIZE_MAX, ' ') to throw std::length_error.
Since the exception is uncaught, std::terminate() is called and the process
aborts.

Fix: add a column > 0 guard before the subtraction.

Reproducer (36 bytes, pure ASCII):
  GL_EXT_shader_image_load_formatted\\n
  → TShader::preprocess(version=450, ENoProfile, EShMsgDefault)
  → terminate called after throwing 'std::length_error': basic_string::_M_create

Author: ashketchumwashere

glslang/MachineIndependent/ShaderLang.cpp

@@ -1178,7 +1178,8 @@ struct DoPreprocessing {
                 // Don't emit whitespace onto empty lines.
                 // Copy any whitespace characters at the start of a line
                 // from the input to the output.
-                outputBuffer += std::string(ppToken.loc.column - 1, ' ');
+                if (ppToken.loc.column > 0)
+                    outputBuffer += std::string(ppToken.loc.column - 1, ' ');
             }
 
             // Output a space in between tokens, but not at the start of a line,