mesa: initialize VertexProgram._VaryingInputs before the first use

Noticed by code inspection.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21438>

src/mesa/main/context.c

@@ -711,7 +711,6 @@ init_attrib_groups(struct gl_context *ctx)
    ctx->NewDriverState = ST_ALL_STATES_MASK;
    ctx->ErrorValue = GL_NO_ERROR;
    ctx->ShareGroupReset = false;
-   ctx->VertexProgram._VaryingInputs = VERT_BIT_ALL;
    ctx->IntelBlackholeRender = debug_get_bool_option("INTEL_BLACKHOLE_DEFAULT", false);
 
    return GL_TRUE;

src/mesa/program/program.c

@@ -89,6 +89,7 @@ _mesa_init_program(struct gl_context *ctx)
    ctx->Program.ErrorPos = -1;
    ctx->Program.ErrorString = strdup("");
 
+   ctx->VertexProgram._VaryingInputs = VERT_BIT_ALL;
    ctx->VertexProgram.Enabled = GL_FALSE;
    ctx->VertexProgram.PointSizeEnabled =
       (ctx->API == API_OPENGLES2) ? GL_TRUE : GL_FALSE;