glcpp: Fix post-decrement underflow in loop-control variable

This loop-control condition with a post-decrement operator would lead to
an underflow of collapsed_newlines. This in turn would cause a subsequent
execution of the loop to labor inordinately trying to return the loop-control
variable to a value of 0 again.

Fix this by dis-intertwining the test and the decrement.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65112

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/glcpp/pp.c

@@ -97,8 +97,10 @@ remove_line_continuations(glcpp_parser_t *ctx, const char *shader)
 			{
 				ralloc_strncat(&clean, shader,
 					       newline - shader + 1);
-				while (collapsed_newlines--)
+				while (collapsed_newlines) {
 					ralloc_strcat(&clean, "\n");
+					collapsed_newlines--;
+				}
 				shader = newline + 1;
 				search_start = shader;
 			}