glsl: Initialize gl_shader_program::EmptyUniformLocations.

Commit 65dfb30 added exec_list EmptyUniformLocations, but only
initialized the list if ARB_explicit_uniform_location was enabled,
leading to crashes if the extension was not available.

Cc: "11.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>

src/compiler/glsl/linker.cpp

@@ -3180,7 +3180,6 @@ check_explicit_uniform_locations(struct gl_context *ctx,
       }
    }
 
-   exec_list_make_empty(&prog->EmptyUniformLocations);
    struct empty_uniform_block *current_block = NULL;
 
    for (unsigned i = 0; i < prog->NumUniformRemapTable; i++) {

src/mesa/main/shaderobj.c

@@ -240,6 +240,8 @@ init_shader_program(struct gl_shader_program *prog)
 
    prog->TransformFeedback.BufferMode = GL_INTERLEAVED_ATTRIBS;
 
+   exec_list_make_empty(&prog->EmptyUniformLocations);
+
    prog->InfoLog = ralloc_strdup(prog, "");
 }